every day. so why did investigators suspect that leslie's death wasn't an accident at all? >> the evidence at the scene makes us believe that something entirely different happened. >> he told me she fell 20 feet. where were the injuries? why did i not see what i expected to see? >> reporter: for investigators, disturbing clues in the woods and water. >> if your theory is right, that is like she was being hunted. >> yeah, it's almost like a hare and a wolf. >> reporter: two harrowing accounts of what happened to leslie. one seemingly un-solvable mystery. >> either you believe crazy story "a" or you believe crazy story "b". both stories are crazy. which one do you want to go with? >> reporter: i'm lester holt and this is "dateline". here's andrea canning with "the secrets of cottonwood creek". reporter: winding down a mountain in southwest colorado, waterfalls spill over granite and sunlight filters through aspens and evergreens. cottonwood creek, an only-in-the-west sort of backdrop. these snapshots barely capture it. but who can resist trying? what a spot to preserve a memory. careful though. this is wilderness, the dangerous kind. >> she's in the river. she fell and they can't find her. he thinks she's dead. >> reporter: the frigid currents here can take a life, and also, maybe, sweep away secrets. >> it was the perfect place to commit a murder where nobody would see you. >> if you go into it looking for trouble, you're gonna find it. and up in those mountains, sometimes questions are left behind. >> i don't have an explanation for this. but certainly, it does truly haunt me. >> reporter: the story begins with those pictures. together they form a couple -- husband and wife -- leslie and fred mueller, married for almost 27 years. >> they were always super affectionate. they acted like they were two high school kids that had just started dating. they're just all over each other. >> and you're like, "ew." >> reporter: these are the muellers' three children -- amanda, the eldest, then ariel, and alex, the youngest and only boy. they were brought up in texas, near the town of san angelo, but for fun the family headed out to their second home, in lake city, colorado. those photos of their parents were taken nearby. >> they both loved the cabin and going up to colorado. >> it was so pretty. it was in a valley, and there were just -- and it would like a postcard, the mountains. >> yeah, there was a river running right near there. >> reporter: their rockies getaway was a family place to relax and escape their busy lives back home. what was life like up at the cabin? >> we would usually go in the summers. >> uh-huh. >> go hiking together, horseback riding, and just kind of hang out as a family. >> reporter: did you feel like there was a lot of family bonding going on at the cabin? >> yeah, definitely. we'd end up having a lot of fun, with board games or going out hiking, and doing all these different activities together. >> reporter: in texas, leslie had an ob/gyn practice, and worked long hours delivering babies. >> and she was the area's first ob/gyn, female ob/gyn? >> she was. she was very much a pioneer, in that sense. >> yeah. >> like, i think the more people told her, "you can't do this," she wanted to do it. >> uh-huh. >> yeah, she was very strong-willed and definitely a feminist in a way, that she was all about, like, "i can do anything that any man can do and i can do it better." >> reporter: in 2004, leslie decided to make a change in her life. she retired from medicine and devoted time to her church. she even studied theology. but, most of all, leslie loved the outdoors, especially riding horses. >> i think if she had her choice, she would be riding horses every day and then practicing theology. and i'm sure she wished the days were longer. >> reporter: yeah, she's like a superwoman. their mom embraced adventure, and she wanted her kids to as well. >> one time mom and i went, just the two of us. i hadn't been horseback riding in a while. and so i told her i didn't want to go more than maybe five, six miles. and she goes, "okay, well, i found this trail. it's eight miles, but you can do it." and by the time it ended, it was 16 miles. >> reporter: so she tricked you. >> yeah, but we had such a wonderful time. but the next day i was not moving. >> reporter: as for fred, he owned a multi-million dollar company importing steel for construction. still, their dad wasn't all business all the time. >> in the traditional motherly roles, he would definitely step up and wasn't afraid to be mr. mom. >> reporter: the mueller kids say their parents fit one another perfectly. >> he'd make a point of every night to try to do something nice for her. >> he was so proud of her, too. >> they'd talk all the time. anytime that there was any type of disagreement about, you know, who -- the dishes weren't done or some "honey, do chore" kind of a thing, they would talk about it until they were completely resolved. and then at the next second they'd be all over each other again and be perfectly happy. >> reporter: it was a happy weekend at the mueller's colorado cabin. may 3rd, 2008. now that leslie was no longer working, she was spending much more time up here. that saturday, the spring day was sunny, the high altitude air still chilled. morning began with chores. >> i think we were unloading some hay. doing stuff kind of like that around the house. >> reporter: so was this just a very typical weekend in colorado? >> yeah. >> reporter: it was just alex, then 14, up with his parents on this trip. that afternoon, the three went to mass in town. later, after a snack, his parents suggested they cap off the day, with a hike to photograph some stunning waterfalls nearby. >> i was pretty wiped out by that time, and i just kind of wanted to hang out in the cabin, and just read a book or something like that. so i turned them down on going for the hike. >> reporter: fred and leslie headed off with her dog gracie, a border collie pup. >> i think she was only six months old. >> rambunctious? >> she was rambunctious, for sure. she's the type of dog that when the ceiling fan would be going, she would try to jump up, and like, follow it around. >> reporter: fred and leslie parked at the base of the trail. snow still covered the ground in patches. they hiked up, past a small series of waterfalls. up until they found the perfect spot for pictures. they had arrived at cottonwood creek. fred struck a pose first. next, leslie's turn. one more of fred. big smile. and then leslie again, this time with her beloved gracie. then, something went wrong. >> it's like it's just happening in slow motion in front of me. >> reporter: in an instant, leslie was gone. >> at the waterfall, she's in the river. >> reporter: what happened at cottonwood creek? the hunt for answers starts with a frantic search. where was she? when we return, the first clues. >> they were yelling and the yelling was coming from downstream. mm. you know what's gone? kevin? the aspartame in yoplait light - they took it out. oh yeah, it is gone. what's wrong with kevin? vo: yoplait light with 90 calories. smooth, creamy, and now aspartame-free. get up to 60% off, plus 10% off. sears mattress spectacular! plus an extra 5% off or up to 60 months special financing, and, free delivery and assembly. start your mattress search at sears. no, no, no... germs, germs, icky germs, icky germs (bell rings) cute cute that's cute. delete it...cute (bell rings) by their second kid, every mom is an expert and more likely to choose luvs than first time moms. oh, i think your kid's eating sand. probably cute live, learn, and get luvs where you can get more from our reliable 4g lte network with a free 4g lte smartphone, after mail-in rebate. it's our gift to you. or anyone in your family, really. hey! gue>>hump day! it is?? 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(sigh) if you want to save fifteen percent or more on car insurance, you switch to geico. it's what you do. ok... reporter: alex mueller was alone at his family's vacation cabin in colorado. his sisters weren't on this weekend trip, and that saturday afternoon his parents went out for a hike without him. do you remember the last words you said to your mom as she was leaving the cabin? >> just, "see you later," you know. >> reporter: just a normal, like, "of course i'm gonna see my mom in an hour." >> yeah, exactly. i mean, nothing really memorable. >> reporter: and so then, there's a whole period of time that they're gone. what's the next thing you remember? >> so reading, kind of hanging around. i think the first time i kind of noticed that it was getting a little late. it had gotten dark and, you know, nobody should be kind of hiking or walking around when it's dark. >> reporter: alex had no inkling of the tragedy unfolding at cottonwood creek. it started right after his dad took this picture of his mother, leslie. leslie and gracie posed here. you can see it's a stunning setting. but just about an arm's length behind me is a cliff. a steep drop off to the granite below. and the icy cold waters of cottonwood creek. right after fred snapped the picture, leslie fell and was swept away in the water. unable to find her, fred raced for help, driving more than a mile to the nearest house. >> somebody just showed up at my house. >> reporter: with no cell service up by the creek, the owner of the house, justin sparks, was the first to call 911. >> he pulled up. he said, at the water fall, she's in the river. she fell and they can't find her. he's frantic, of course. he said he thinks his wife's dead. >> it's his wife? >> yes. >> reporter: the homeowner offered right away to help fred. they sped back up the mountain and fred dropped him off at a spot to begin searching. just down the road, physicians assistant and ems volunteer michael golub was on duty. >> a call went out over the radio for a female drowning victim. >> reporter: golob headed over to the base of the trail. there he ran into fred. >> he comes barreling down at a very high rate of speed. kind of slams on his brakes and starts yelling at me, "let's go, let's go, let's go." >> reporter: the ems responder noticed this man he was trying to help could also be injured. fred's face was scratched up. >> he has some superficial lacerations. and so i take a minute to make sure this isn't my second victim, to make sure he's okay. and he assures me he's fine, he's fine, just kind of wants to keep going. >> reporter: but then as fred drove, he seemed disoriented. he pulled over, but he couldn't seem to find where leslie fell. >> i start to unload from the jeep. he says, "no, this isn't it." and we get back in the jeep and he continues upstream or uphill. >> reporter: after another stop, they found the right place, and hiked toward the creek. >> all of a sudden, i hear yelling, and the yelling is coming from downstream. >> reporter: it was the homeowner justin sparks. michael golub took off toward him. he was in a knee-deep pool down below the waterfalls. >> there was mrs. mueller. she's dressed in a lime green jacket. and she's floating face up. >> he actually found her. i believe he used the word pinned under the log and face down. >> reporter: leslie had no pulse. but golob thought there was still hope that she could be revived. fred had caught up, towering over them from the creek embankment. >> he says, "is she dead?" i tell him it's too soon to know. >> reporter: golob sent fred back down the road to look for more help. by then, a small village of first responders had assembled at the start of the trail, among them was hinsdale county sheriff's deputy justin casey. >> we had other officers responding, that we also had the ems personnel coming in ambulances, and we had fire personnel coming with the fire trucks. >> reporter: dispatch put a call into the local sheriff, ron bruce. >> we had gotten a report of a drowning up on cottonwood creek. and they said they'd keep me advised. >> reporter: and this probably didn't seem totally out of the ordinary for you? >> no, because we've had them before. >> people fall on -- when they're just simply hiking, they break something. >> and some people die up there. and we have to retrieve the bodies. >> reporter: by then, a group of medical responders made their way up to the creek to help. but still, no heartbeat, not a breath from leslie. and it was clear, she was gone. >> we were not going to be able to bring miss mueller back. >> reporter: at the cabin, alex was wondering why his parents weren't back yet. their hike was only supposed to last an hour or so, but they'd been gone more than three hours. were you worried? >> not really worried, as in something happened to them, just more like, it's kind of weird that they would, you know, stay out past dark if they're going for a hike. >> reporter: it was past 8:00 p.m. when his dad got home without his mom. >> it's my dad, and the deputy, and -- yeah, they come in. it's kind of a shock to see, you know, when you're expecting somebody, to see strangers coming in instead. >> reporter: what do you think when you see that? >> not a whole lot of time to think, honestly. you kind of go into shut down mode. >> reporter: alex recalled his dad seemed like he was in shock. >> he was very disheveled, had been crying a lot. and then, he, you know, gave me a big hug, and that was kind of how it broke. >> reporter: i can't even imagine getting that news, when you've just thought your parents were going for a quick hike. >> just kind of disbelief, really. the whole thing just seemed so surreal. >> reporter: then, father and son were shuttled to the sheriff's department, where fred could file a report about the accident. >> i remember them saying, "you take fred. i'll take alex." >> reporter: sheriff bruce was at the station to meet them. >> my initial reaction to fred was he was a victim, a grieving husband. >> reporter: fred gave a brief, hand-written statement. >> we want to make sure we've dotted the i's and crossed the t's. we were dealing with the death of a woman. >> reporter: then, fred and alex headed back to their empty cabin. and there, in that home that had, until then, been full of such happy memories, fred and alex endured the longest night of their lives. >> we stayed together in the same room the whole night. >> reporter: and did your dad sleep at all that night? >> it was a very long night. >> reporter: but they had no idea it was just the first sleepless night in what would become the longest ordeal of their lives. coming up -- fred gives investigator as details account of what happened to leslie and he's very clear on who's to blame. >> it it was dog tangled up in her legs, is what i think it was. >> what would police think of that when "dateline" continues. wow, i've got a lot of stuff to carry. you are more powerful than you think. ♪ eh, yea, actually i do. one. it's mucinex fast-max night time and it's got a nasal decongestant. is that really a thing? it sounds made up. mucinex fast max night time for multi-symptom relief. breathe easy. sleep easy. let's end this. which means it's timeson for the volkswagen sign-then-drive event. for practically just your signature, you could drive home for the holidays in a german-engineered volkswagen. like the sporty, advanced new jetta... and the 2015 motor trend car of the year all-new golf. if you're wishing for a new volkswagen this season... just about all you need is a finely tuned... pen. hurry in to the sign then drive event and get a five-hundred dollar new year's bonus on select new volkswagen models. offer ends january 2nd. reporter: fred and leslie mueller's daughters, amanda and ariel, were hundreds of miles away from colorado when it happened. their dad had to break the terrible news to them by phone. >> he said, "i'm so sorry. i'm so sorry." and i said, "what happened?" and he said, "we were hiking. i was trying to take a picture. she fell." and i remember saying, "but she's going to be okay." and he said, "no, she's dead." and i kind of lost it. i started crying terribly. and he said, "i'm so, so sorry, mandy." >> reporter: what did your dad say to you when you got him on the phone? >> i don't even think i asked him what happened, because you don't want to hear details. i mean, i could easily say that was the worst day of my life, that i found out about my mom. and all it -- the only detail that matters, that she was gone. >> reporter: the tragic details, however, were the business of hinsdale county sheriff ron bruce. he and his small office have the huge job of patrolling more than 1100 square miles of potentially dangerous wilderness and wildlife. >> should they have not gone -- >> oh no, no. it was perfectly fine. i think he related it was something of an idealic day. >> reporter: the day after leslie died, sheriff bruce stopped by the muellers' cabin to get a more precise account of what happened during their hike. bruce's undersheriff and a state investigator talked to fred. >> you know i shouldn't have even taken her up there. it's all my fault. >> reporter: the conversation at the muellers' kitchen table was recorded. >> we started taking photos. she took a few of me. i took some of her. >> reporter: fred described in detail how that kodak moment went wrong. >> i suggested that she take a picture with her dog and it's a border collie. and it's a very -- it's just extremely skittish dog. she looks at me, and i take her picture, and we think everything's cool. and, i think like a bird kind of flutters by. >> reporter: fred said the rambunctious border collie got startled. >> but the dog just jumps out as she's turning. and it's like her feet just went out from under her. it's like it's just happening in slow motion in front of me. she falls forward and, and i remember lunging forward to try to get to her but i was probably five, six, seven feet from her. >> reporter: fred said he witnessed leslie free fall. >> it looked to me like she just did a swan dive. and, and lands on the rocks, right by the water. just, just, like head and shoulders, and just crumples. and then just, just slides like mush into the little channel. and i'm screaming her name. i'm hollering. i should've just jumped in. >> reporter: fred said he lost sight of leslie in the water so he ran down to where he thought the current would take her. he said he got scratched up as he stumbled up and down this embankment navigating rocks and running through bushes. >> i must have tripped. i don't know a few times, quite a few times coming down. and i'm sure i hit obviously hit something coming down a few times. >> reporter: he told how he had left the camera with those last images of his wife behind. >> i would imagine i just dropped it while i was screaming. >> reporter: unable to locate leslie, he abandoned his search of the creek bed and made the decision to go for help. >> i hauled butt down to justin's house. and i'm honking, and i scream at him to -- you know, i think my wife's dead. >> reporter: after listening to fred's story, the sheriff wanted to go up to the site of the accident. you take fred back up to the mountain? >> yes, we did a walkthrough on the scene. >> reporter: up at the scene, fred repeated how the dog seemed to cause leslie's fall. >> it was that damn dog tangled up in her legs, is what i think it was. >> reporter: gracie ran off into the woods right after the accident. a member of the sheriff's department later found her and took her back to his house. >> everything changed that day. >> reporter: changed forever, in ways the kids could not, in their immediate grief, begin to grasp. that's because, while their father was on that mountain talking to authorities, seeds of doubt were being planted, right in the landscape around cottonwood creek. coming up -- one of those seeds found near the ledge where leslie spent her last moments. >> all the alarm bells were going off tochlt this day, it gives ne chills. is all the end-of-season deals on great tech stuff at verizon. the not so good more, having the season actually end. now get the amazing moto x in football leather free. quitters, you're all quitters! and get $150 credit when you switch to verizon. ♪ vicks nyquil severe. helps relieve your ugliest, nastiest, roughest, toughest cold symptoms. vicks nyquil severe. with maximum symptom fighting ingredients. ♪ don't disguise bad odors. ♪ continuously neutralize them with glad with febreze. get up to 60% off, plus 10% off. sears mattress spectacular! plus an extra 5% off or up to 60 months special financing, and, free delivery and assembly. start your mattress search at sears. wall isn't a street... return on investment isn't the only return i'm looking forward to. for some, every dollar is earned with sweat, sacrifice, courage. which is why usaa is honored to help our members with everything from investing for retirement to saving for college. our commitment to current and former military members and their families is without equal. start investing with as little as fifty dollars. reporter: fred mueller spent the day after his wife's death talking with investigators about the accident on cottonwood creek. the next day he and his son alex flew home to texas, ferried by a friend in a private plane. >> you never think you would -- it could happen to you. >> reporter: there, fred and alex were reunited with amanda and ariel, alex's older sisters. >> you hear horrible stories and you feel for these people. it was surreal that we were now that family, and now -- you know just trying to get through it. >> reporter: and as they spent their days preparing for their mother's memorial, the family took some solace in how loved leslie had been and they knew why. >> she was a firecracker. she was very loving, very loyal. >> brilliant. she's probably one of the smartest people that i've ever met. >> very smart. >> or ever known. >> reporter: so many came to pay their respects. but mourning took its toll. >> i would hold myself together in front of people who would visit, and then i would go in the other room to cry, and break down. >> which is natural, yeah. >> and it's kind of just saving face a little bit. >> reporter: leslie, the horse lover, was buried on the texas ranch where she grew up riding. when the blur was over, and you finally had time to process what had happened, what did you think about with your mom and your future? >> i was angry. i was really looking forward to being pregnant and calling my ob/gyn mom any time i had any kind of little, odd thing happen. >> it was like two weeks before my graduation. it was like a week before my prom. like all these big things that weren't the biggest things in the world. but it was just -- like, things that your mom should be there for. and i mean -- seeing us grow up. >> reporter: the kids weren't just grieving their own loss. they felt their father's too. >> i had never seen him so sad, or -- you know, you just didn't know if he was gonna do something crazy, or -- you know, if he was ever gonna get over this, and be able to move on. >> reporter: and up in colorado, sheriff bruce wasn't moving on either. he still had questions about the accident and fred's story. he'd sent deputy justin casey up to cottonwood creek to document the site. casey brought his brother with him. they snapped photos and shot video along the way. >> we started below where mrs. mueller's body was found and took photographs below as we progressed upstream. >> reporter: as the casey brothers moved up the creek, they noticed that the creek's flow didn't seem very strong. >> it didn't push us around as we were walking. we were careful with our footing in the sense that there are slippery rocks underneath the water. but it wasn't hard to go against the current. >> reporter: the deputy couldn't help but wonder, how had leslie's body floated so far. >> the water is coming down. but it's not a raging river. it's not fast moving. >> reporter: deputy casey reported back to sheriff bruce, who by that time had talked to the couple who lived by the trail and had helped fred in those first, frantic moments. what they said cast a new light on fred's story of that awful day. >> little tickling at the back of their neck had sent up some red flags to them. >> reporter: they told sheriff bruce that, as they called 911 and set out to help in the search, fred was frantic one minute, then seemed calm the next. >> they said the vibes were all wrong from him. they became very uncomfortable. >> reporter: did they kind of, like, in hindsight think back or was it right then? >> they were immediately concerned that something was amiss. >> reporter: it was the husband, justin, who found leslie's body in the creek, but he told the sheriff that initially fred had sent him off to search too far downstream. >> he drove up a short ways and said he was sure that his wife's body was somewhere in that area. justin thought that was odd. >> reporter: was it possible that fred was disoriented? it's kind of a tough area to navigate? >> it's possible. >> reporter: yet sheriff bruce said he too got a strange vibe from fred the night leslie died. and again, the next day, when they went back up to cottonwood creek. >> his emotions and his responses to everything seemed to be extremely flat, and i thought almost rehearsed. >> reporter: is that fair, though? with everyone being different in this world and everyone reacting differently to tragedy, is it fair for you to judge him based on how you think he should be acting? >> i don't know if it's fair. but in my business, it's realistic. because you have to look at all angles of an event and make sure that you're uncovering everything that's there. >> reporter: on a ledge near where fred said leslie fell, investigators found fred's camera and the photos of leslie's last moments alive. as he stared at leslie, the vibrant wife and mother, sheriff bruce's gut told him keep looking. >> the vibes were all wrong. all the alarm bells were going off. even to this day, a little bit gives me the chills. coming up -- fives are one thing. investigators thought they had evidence that fred wasn't telling the truth. >> he told me she fell 20 feet. where was the trauma? where were the injuries? why did i not see what i expected to see? >> when "dateline" continues. noth-- my wedding ring. 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[ high-pitched ] nailed it! about adding up all the taxes and fees to your monthly bill. that's why our plans start at just $35 bucks, after $5 auto pay credit, all in. taxes and fees included. i to the acidity in any foods.ht never thought about the coffee i was drinking having acids. it never dawned on me that it could hurt your teeth. he told me to use pronamel. it's going to help protect the enamel in your teeth. it allows me to continue to drink my coffee, and it was a real easy switch to make. reporter: for the folks charged with keeping the woods and waters around cottonwood creek safe, leslie's mueller's drowning was a tragedy, but also a mystery. >> you know i put myself in fred's shoes and they didn't fit. >> reporter: sheriff ron bruce had thought fred's demeanor the day leslie died was unusual. fred, to him, showed a lack of emotion. but what really spoke volumes to the sheriff and his team, was something they knew well, the terrain around cottonwood creek. as fred told them, leslie did a swan dive from up there, landing on her head and shoulders on this rock. then she slid into the water. a horrible accident. but to local law enforcement, something seemed off. >> reporter: the first trained rescuer on the scene noticed it first. what appeared to be a major inconsistency in that story, after such a treacherous fall, leslie's body, when pulled from the creek, didn't look at all injured. >> he told me she fell 20 feet. where was the trauma? where were the injuries? why did i not see what i expected to see? >> reporter: one look at that drop off, and sheriff bruce was wondering the same thing. >> and when the autopsy report came back, it only heightened his suspicions. >> there were no broken bones, any substantial abrasions or contusions. they simply weren't there. >> reporter: every accident is different, every fall is different. is it possible that she just fell in a certain way that just didn't produce those injuries you were looking for? >> no, if she landed head first, as he told us she landed, she would have had head injuries, facial fractures, spinal trauma. none of that was present. >> reporter: so sheriff bruce made a quick conclusion. to him, no injuries, meant no fall. fred had to be lying. we went back to cottonwood creek with the sheriff in may of this year, the same month leslie died. >> we got a late snowstorm this year. >> reporter: a spring storm had just blanketed the woods and rocks. >> i have to say, i'm honestly nervous just standing here. this is very steep. >> that's a sheer drop-off. >> reporter: it was up here where they had posed for pictures, that sheriff bruce showed me what he thought were key pieces to their puzzle. >> the evidence here at the scene makes us believe that something entirely different happened. >> reporter: they found trampled bushes, what they thought looked like scuff marks, and a pair of broken glasses that turned out to be fred's. >> there was evidence of a struggle in there. and that's where fred's mangled glasses were found. >> reporter: sheriff bruce formed a theory that, after snapping those photos, fred and leslie got into a physical fight. >> we think that at that point -- i think, anyway -- that she was able to break free of his grasp at that point. and she ran back this way and down, paralleling this path that goes down eventually -- meets up with this stream. >> reporter: you think he chased after her? >> i think he paralleled her down that road until he got to the point where he could easily intercept her as she's going down by the stream. >> reporter: if your theory is right, that is almost like she was being hunted. >> it's almost like a hare and a wolf. >> reporter: and according to bruce's theory, fred chased his wife right into the shallow pool where she was found. >> that's where he held her underwater and drowned her. and then, placed her right by the submerged log. but bruce admits, investigators lacked some evidence to back up that scenario. what about footprints? there were no footprints in the mud or -- >> no. and by that time, we'd had investigators in here, and it'd been tracked up pretty good. >> reporter: so the evidence was ruined essentially? >> yeah. >> reporter: if there was evidence? >> right. >> reporter: the sheriff and agents from the colorado bureau of investigation, the cbi, worked together, investigating what they now thought was a homicide. they went down to where the muellers lived in wall, texas, to poke around in fred's life, but they couldn't find anyone to say anything bad about him. >> we found that the interviews were largely unproductive. >> reporter: you were hitting walls? >> in wall, texas, which is an outlying community outside of san angelo, it's a pretty closed community. >> reporter: and inside that community, none of leslie's friends or family doubted fred and couldn't understand why folks up in colorado were asking questions. >> yeah, people loved dad. that's why he had immense amount of support. >> reporter: and then, ten months after leslie's death, fred mueller got an invitation from the cbi. an agent wanted fred to come back up to colorado for a talk. >> he casually mentioned, like, "oh yeah, i'm going back to colorado. they wanted me back up there." he's just saying, like, "you know, i don't know what they're doing, either. but it's just -- we got to cooperate. they're doing their job. let's just get past it so we can move on." >> i just wanted to first fred thank you for coming. >> you're welcome. >> reporter: again he shared his recollections about that day at the vacation home. >> it was a beautiful day and it went to hell. there are so many things i wish i had done differently. >> reporter: and again described what happened to leslie after she posed for that last photo. >> she was flailing in the air, screaming. and i was screaming and she hit, and never made another peep. >> reporter: if fred's account of the accident was the same, the tone of this interview was different. the cbi agent made it clear they didn't believe his story. >> and you know i want to take this opportunity to ask you everything that's making me think that this may not have been an accident. >> ask me anything you want. >> reporter: they questioned fred about those broken glasses found at the scene. >> i'm just gonna say it. i'm thinking you two had an argument by this bush. >> no ma'am. no ma'am. my wife and i did not have an argument anywhere that day. >> because you -- >> that was the best day of our lives until that accident. >> reporter: the best day of their lives? the agent clearly suspected there had been trouble in the muellers' marriage, a suggestion fred cooly dismissed. >> i can see after 27 years the two of you grow apart. >> no. i think we made a hell of a good pair. >> and you don't know quite how to deal with that. >> or -- or -- >> or? >> one could say that after the 27 years we were looking forward to retirement. >> reporter: and fred insisted that on the day she died, he found her more attractive than ever. >> she was beautiful and she knew that. and i would tell her that. she became more beautiful from being already beautiful in my eyes, became more beautiful as she became older. and we had a very -- that's probably none of your business. we had a very active sex life and we got along great. and if y'all did a good autopsy you would know that. you would know that. >> what are you trying to tell me? >> that we made love that morning. >> reporter: but even as the seemingly confident fred grew clearly irritated, the agent wasn't deterred -- what about those scratches on his face the day leslie died. >> so these vertical scratches you're saying you got from bushes? >> bushes or that ground or running down this hill. falling a few times, which i did fall, but it had to have been from some of that. there's no other explanation. >> okay, well then -- >> that's fact. that's factual. >> there was no altercation up there? >> no, ma'am. no, ma'am. >> because it looks -- >> check underneath her finger nails if you think there's something under her nails that's missing off my face. it didn't happen. there was no altercation. write it down. none. >> reporter: for now it seemed that was all the agents could do, document fred's story and let him leave. >> i'm sorry to be a crude open book. >> time is 11:23. i am turning off the recording. >> reporter: investigators had their theories, but did they have a case? fred went home to texas where his family thought would be the end of it. >> his entire philosophy through all of this was it was an accident. i'm telling the truth. >> and he wanted to cooperate. he wanted to help them see that. >> reporter: but investigators hadn't seen that. not at all. and if fred couldn't answer their questions, they'd head back to cottonwood creek and hope that the water could. coming up -- fred has always said that water carried leslie away to here, where her body was found. now, investigators put that story to the test. >> they said there was no way her body could have been under that log unless it had been placed there. when the game's on the line. hit him with a hard count, see if they'll tip their hand. the nfl trusts duracell quantum to their game day communication. they're blitzing up the gut! get out of the pocket! hut! duracell quantum. lasts up to 35% longer than the competition. sir, we're going to need you on the runway. 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(vo) theraflu. serious power. >> reporter: time went by and the muellers didn't hear anything from the colorado investigators. the kids assumed the sheriff and his team had finally decided to believe their dad too. >> we didn't hear from them for three years after that. and so i thought that -- >> i just thought it was done. >> yeah, we didn't -- >> i guess they finished up their -- >> crossed their -- >> -- paperwork or whatever they needed to do. >> and then it was over. >> reporter: it wasn't over. >> i am an open book. >> reporter: in fact, after fred's interview with the cbi, investigators kept working. what drove them was their theory about a possible struggle, and that curious lack of injuries on leslie's body. but the sheriff knew they needed more, and he put that last photo of leslie behind his desk. >> she was my constant reminder. and that photo was what was -- what was the nudge that kept me going on this case. >> reporter: investigators decided to zero in on one aspect of fred's story - - was it really possible for leslie's body to float from where he said she fell in to where she was found, 150 feet and three waterfalls downstream? >> when we brought in the really top-notch water flow experts, we had to go back and try to duplicate the water flow as it was on the evening of the event. >> reporter: in august 2009, fifteen months after leslie died, investigators headed up to cottonwood creek to conduct some tests. they dressed a woman, around leslie's size, in similar clothing to what she wore the day she died. and positioned the female stand-in on the rock ledge where fred said leslie landed before sliding into the water. >> the forensic people tried to float her from that location, where - where he alleged that she fell. the water did barely -- barely touched her body. >> reporter: so, investigators put her in the water to see what would happen. >> she was then moved into the very center of that stream and it wouldn't budge her. >> reporter: finally, they went down to the pool where leslie was found. >> they then put her in a wetsuit, which gives her buoyancy. >> reporter: she was able to float. but unassisted, she had a hard time making it all the way to the log where leslie's body had been pinned. this meant something big to the investigators. >> so, it belied what he told us about this raging torrent tearing her away from his view. >> reporter: they tried it all again with a mannequin, and again, what they saw did not match what fred recalled in the deeper pools the mannequin sank. and it got stuck at the falls. >> each time she had to be physically moved over those -- the lip of those waterfalls, the water -- the stream would not move her. >> did you feel like because this case was hard enough as it was, that this testing had to be just as perfect as it could be? >> of course. with no witnesses, we're dealing strictly with circumstantial evidence. and -- and circumstantial evidence is often enough to get you a conviction. but we knew we needed to have a lot of it. >> reporter: to investigators, these tests confirmed their suspicions -- to them, fred's story of leslie's accident was impossible. >> they said there was no way that her body could have ended up under that log unless it had been placed there. that the forces of nature would not have put her under there. >> reporter: but were these tests enough to make a full-fledged criminal case? the first person they'd have to convince was the local district attorney. >> he was quickly brought on board as to what we were investigating and what our thoughts were. >> reporter: it turned out to be a hard sell. >> for whatever reason, we could never peak his interest. he never said no, but he just -- i didn't find the enthusiasm. >> reporter: even after those tests, the d.a. wasn't ready to press charges. months went by, then a year. leslie's picture was still there right behind sheriff bruce's desk, but the investigators weren't about to shelve their case. >> are there things happening? are the wheels turning? >> it was constantly on our radar screen -- that there were things yet to be done. but the months did run on. >> reporter: all this time, fred was a free man in texas. >> his business ventures were -- were moving on. and -- and at home, we were all saying, "fred mueller believes he got away with murder." >> reporter: but sheriff bruce says fred mueller may have underestimated the hinsdale county sheriff's department. >> i think that was he -- what he thought. he was gonna get some kind of bumbling buffoon to show up. and he was gonna walk away on this. >> what do you say to someone who says, "you're a small-town sheriff, who got tunnel vision, who focused on this rich outsider and just like a dog with a bone would not let go"? >> i -- well, i'd say they're wrong. and i -- and i've got thick skin. they're -- we did the right thing. >> reporter: they kept working and testing and poking around, not sure if it would ever amount to anything. to go forward, something needed to change and it was about to. coming up -- the sheriff makes his case to a new d.a. everything changes. >> we were able to get 30 minutes of his time thchlt is what occurred. he said let's go. >> and for the kids, a shock. >> i was like why. they said we can't tell you that. >> a new chapter opens. nothing in this case will go the way anyone things, when "dateline" continues. bounty. the long lasting picker upper and now try new bounty nfl prints. available at walmart. welcome bigger savings at the sears year end event. use your sears card and get an extra 15% off all appliances. or 36 months special financing on appliances over $499. sears. ♪ but the fire is so delightful ♪ [ male announcer ] nothing melts away the cold like a hot delicious bowl of chicken noodle soup from campbell's. ♪ let it snow, let it snow returning to our story, after a romantic hike with her husband, leslie's body has been found in a pool of water. fred says she fell off a cliff investigators have problems with fred's story and believe he killed leslie, the d.a. refuses to bring charges. now, a new d.a. comes to town. everyone involved in the case will be in for a shock. again, here is andrea canning. >> reporter: as the years moved on since leslie mueller's death, investigators were continuing to build a case against fred, though his children were unaware their father was under such intense suspicion. they were trying hard to remember the happy moments with their mother and put the colorado tragedy behind them. >> what became of the cabin that held so many memories for all of you? >> i think after the accident, it wasn't gonna be a happy place for us anymore. and so dad decided to sell the place shortly after mom died. >> i would imagine that whole area's just kind of bittersweet now. and -- >> unfortunately, it's a beautiful area. it's just -- >> not for us. >> reporter: for the mueller children, life seemed to be stabilizing. amanda was settling in with her husband. ariel was in college. and alex, a high school student, was dreaming of a career in the navy. but, as alex said, his dad was having a harder time of it. >> he married mom when he was 18 or 19. he depended so much on my mother, and then to all of a sudden be kind of just cast adrift. >> reporter: eventually, fred's counselors and friends suggested he was ready to meet someone new. >> he finally started getting back on his feet and started thinking about moving on, and he met wendee. >> reporter: wendee and fred were introduced through old family friends. >> she came into a very tough situation. but she also, she brought my dad out of a very dark place. you know, she saved his life, essentially, 'cause he was so depressed. >> reporter: after two years as a widower, fred married wendee. >> she was really great and understanding and helped him move -- move forward. >> reporter: a fresh start such a long way from fred's tough grilling in colorado. he had good reason to think it was all over. >> there were times where we were real frustrated. when they say the wheels of justice turn slow, we felt like they were turning too slow this time. >> all the while, as the years are passing, you're looking at leslie's photo in your office every day. >> yes, i am. >> is that keeping you going, focused? >> absolutely. >> reporter: then, in 2011, two and a half years after leslie's death, a new d.a. took office and the sheriff lobbied him to take action. >> we were able to get 30 minutes of his time and -- and say, "here's what we're looking at. here's what we think occurred," he said, "let's go." >> reporter: the d.a. assembled a team to look into the case. matthew durkin, a deputy colorado state attorney general came on board. he was later joined by ryan brackley, on loan from the boulder county d.a. >> what was your first impression? >> very tough case. >> reporter: they combed though the case file -- fred's statements, leslie's autopsy and all those tests conducted at the creek. >> this location was a breathtakingly beautiful place in the middle of nowhere. it was the perfect place to commit a murder where nobody would see you. >> reporter: fresh eyes in the d.a.'s office began to see what the sheriff had suspected from the start. >> the defendant's story did not match up with the landscape, the -- the scene and all of the other evidence that we were -- that the colorado bureau of investigation was able to develop. >> at what point did you finally, all of you, say, "we got enough. let's go get him"? >> i think it was when we felt like we had tested this thing to death. that we had absolutely everything we could possibly put together to take to the jury. >> reporter: february 6, 2012, just over three and a half years since leslie mueller died. a monday -- fred was at his office when a texas ranger showed up with an arrest warrant in hand. sheriff ron bruce, down from colorado, stood a few feet away. >> he looked at us almost in shock. and he said, "i can't believe you people are taking this serious." >> how was that for you to finally make that arrest? >> it felt great. it felt great. and at that point, i said, "leslie, we're gonna make this right." >> reporter: alex was in school when he was suddenly pulled out of class. >> you know, there are some texas rangers here to talk to me. and, i kind of asked, you know, what's going on? why are you really here? "your father's been arrested." >> did you say anything back to them when they told you? >> i was just, like, "why?" and they said, "oh, well, we can't tell you that. we just have some great proof." and i said, "well, it took you four years. what exactly are you doing?" and they said, "well, we just can't tell you that." >> did you ever think for one second, "well, there must be something. he must have done something -- >> no. >> -- for him to be arrested, to be going through this?" >> no, never. >> we never doubted his story. we never doubted it. we never thought that he would've done anything wrong or anything to hurt mom, ever. that's something that i can't even fathom, 'cause he -- >> exactly. >> -- would've never done anything like that. >> reporter: the kids believed the authorities had it in for their dad and trumped up things that weren't true, like that their father showed no emotion after their mother's death. >> he was obviously panicked. he was -- he was panicking. he was shaking. he had been crying profusely. >> reporter: remember, only alex was in colorado at the time and, thinking back, he got angry about the way he and his dad were treated. >> they separate us, like you would with criminals. separate myself from my dad, put us in separate vehicles. >> reporter: to the kids, it all pointed to a rush to judgment that turned into an obsession. >> if you go into it looking for trouble, you're going to find it. >> we were also as a family never informed what they were doing at cottonwood creek, that they were paying for experts, doing all kinds of -- >> we didn't know what was going on. >> -- tests on the water and all these different things like that. but this was all like a closely guarded secret that they never felt they should share with either him or the family, of the victim, of my mother. >> reporter: fred mueller spent the next year in jail in gunnison, colorado, awaiting trial. >> he sunk into such a deep depression right after mom died, and he kind of sank back into it while he was sitting in jail. >> and no one could get to him. it was more the fact that -- >> and we couldn't help. >> he was completely cut off. >> isolated. >> -- from his family for a year. >> reporter: the kids were eager for their dad to take on his accusers. attorney roger sagal was part of fred's legal team. >> after reading through the discovery and the evidence, what were your first thoughts? >> why was this guy arrested for murder was my first thought. it was a question from the very beginning as to what, what evidence did they really have? >> reporter: he'd soon find out in court. more than four years after leslie drowned in cottonwood creek, fred mueller would stand trial for murder. coming up -- at trial, the prosecution challenges fred's entire story, including, stunningly, the one thing that never seemed no doubt, his rock solid marriage. >> fred was telling people the marriage had gone stale and he did have a wondering eye. i like it when my hygienist lets me know i'm doing a good job. i like it when my toothpaste lets me know too. that's why i went pro. go pro with crest pro-health. for an intensive clean. i can really feel it deep cleaning my mouth. for a 4x better clean try these products together. that was a great check up. when electricity is generated here's awith natural gasu: instead of today's most used source, how much are co2 emissions reduced? up to 30%? 45%? 60%? the answer is... up to 60% less. and that's a big reason why the u.s. is a world leader in reducing co2 emissions. take the energy quiz -- round 2. energy lives here. >> reporter: four and a half years after leslie mueller's death, her husband fred's murder trial began in gunnison, colorado. their children, and fred's new wife, dropped everything to be in court and to pray that their dad would soon be cleared. >> are you all just anxious for this trial to start? >> yes, definitely. >> are you on pins and needles? >> i want to see why they put us through this. what do they think that they have? >> reporter: as the trial got underway that frigid january, 2013, inside the courtroom the prosecution felt an icy chill. >> normally when you walk into a courtroom during a murder trial, it's very divided. i walked into that courtroom and everybody was on fred's side. leslie's family included. was that daunting to you at all? >> i wouldn't say daunting. it was certainly unique. but we had to stay focused on what we thought -- what we needed to do. we believed that the defendant was responsible for his wife's murder. >> reporter: it would all come down to two competing versions of what happened here. fred's story -- leslie accidentally went over this cliff and was swept away by the current. or the prosecution's -- that fred was lying to cover up leslie's murder. >> two people went on a hike, and only one person came back. >> reporter: the prosecution began its case with testimony about fred's behavior that day. they called jennifer sparks, whose house fred drove to looking for help. >> did you observe him become emotional at any time? >> no. >> reporter: she told the jury fred's lack of emotion concerned her. >> what conversation did you have with your husband at that time? >> i told him something's not right, be careful. >> reporter: her husband, justin, testified that fred's demeanor seemed staged as they drove together to look for leslie. >> it sounded like somebody was crying but there were no, there were no tears. he acted like he was very, very upset and then a few, few seconds later he'd act like we were kind of buddies driving down the road in a car. >> reporter: the prosecution had hoped to bring the jury up to cottonwood creek. but in january, the terrain was virtually impassible with snow and ice. so, it was up to the prosecutors to set the scene and show why they thought fred's story defied common sense and science. >> injuries are going to tell you a story that leslie mueller was drowned by her husband. >> reporter: prosecutors called michael golob, the first trained rescuer on the scene. >> i'm asking him what happened, tell me how, what happened. and he relates that she apparently had fallen off a waterfall. >> reporter: fred told golob the same thing he'd told investigators -- he'd watched leslie swan-dive off the cliff, hitting granite, before sliding into the water. >> all of a sudden i was struck with the fact that this didn't, there was no trauma. >> did you see any blood on leslie mueller? >> there was no deformities. there was no blood. >> reporter: even leslie's clothing seemed unscathed. >> did you notice any markings, tears, scrapes on the jacket? on the coat? >> there was none. it was remarkably pristine. >> reporter: the pathologist who performed leslie's autopsy was also surprised by the lack of injuries to her body. >> did you observe any injuries to mrs. mueller that were consistent with breaking a fall? >> no, i did not. >> did you observe any consistent with falling on her head and shoulders off a cliff onto granite as described to you? >> no. >> reporter: and what's more, investigators found no forensic evidence that leslie ever hit that rock. >> there was no fibers on this rock. there was no blood on this rock. there was no hair on this rock. >> this is a very unique case in the sense that the evidence is lack of evidence. there's no evidence. >> that's the tough part of the case. what we have is a lot of evidence that what the defendant said is impossible. >> reporter: and here was the heart of the prosecution's case -- those tests investigators conducted up at the creek, tests they said proved fred's story was a lie. they showed the jury the video of those tests, pointing out in one how the female stand-in wasn't swept downstream, and in another, the mannequin wasn't washed over the waterfall and eventually sank to the bottom. >> are you seeing any sliding of the mannequin into the water? >> no. >> any washing or oozing? >> no. >> reporter: the prosecution's drowning expert was blunt. to her, fred's story was pure fiction. >> the water, there's nothing in that environment water-wise that could've gotten her to that spot. a human being had to physically put a body in that position. >> reporter: so, what really happened? in court, the prosecution could only hint at the dramatic story the sheriff told us. his theory of a struggle, a chase and a drowning. well, there were no witnesses for any of that, so it was never actually presented as evidence. but an investigator who searched the area where fred said lesley fell did testify about finding fred's broken glasses. >> this side was broken, the left hand side was sticking in the dirt. >> did you see anything on the ground? >> four foot from the glasses there were what appeared to be scuff marks as if somebody had slipped and had moved the dirt. >> reporter: and on the stand, this witness, the coroner -- >> those are the gloves that leslie was wearing. >> reporter: -- tried to connect those scratches on fred's face to the plastic bumps or nubbins on leslie's gloves. >> to me, those plastic nubbins are a lot more consistent with the scratch marks on mr. mueller's face than branches and twigs. >> reporter: however, after a defense objection, the judge ruled the coroner's statement conjecture and told the jury to disregard it. so, even though the prosecutors were not allowed to tell the jury how they thought the murder happened, they still had a theory to present for the why. >> they were high school sweethearts. he had never abused her to anyone's knowledge. he had no life insurance policy out on her that he was trying to collect. what is the motive? >> fred was telling people -- one being an employee of his, another being a close friend -- that in fact the marriage had grown stale. that it was the same old, same old between he and leslie, and that he did have a wandering eye. >> reporter: and according to the prosecution, his eye had wandered here. >> we were very good friends. >> reporter: jeannie barnes, fred's former assistant, testified that fred called and texted her a lot outside of work. >> did that ever make you uncomfortable? >> a little bit. >> reporter: and once fred talked to her about leslie and divorce. >> he had just kind of mentioned that their interests had kind of grown apart. >> reporter: so, according to the prosecution team, fred picked an idyllic spot in the middle of nowhere to find a way out of his marriage and then concocted the story of her fall as a cover. >> two people went up, one came back. and we've only got his story. and we know that story is false, through and through. >> the defendant has asked people to believe what is impossible. impossible! >> reporter: but fred's lawyers were about to turn the state's case on its head. those tests at cottonwood creek, in fact any so-called evidence of murder just didn't hold water at all. coming up -- defense experts tell the jury the way leslie died is absolutely clear. >> what was it in my opinion? accident. that's what i would put on the death certificate. >> when "dateline" continues. ♪ ♪ even if it's your last one, always share the softness of kleenex tissues. where you can get more from our reliable 4g lte network with a free 4g lte smartphone, after mail-in rebate. it's our gift to you. or anyone in your family, really. welcome bigger savings at the sears year end event. use your sears card and get an extra 15% off all appliances. or 36 months special financing on appliances over $499. sears. er hair? put pantene to the test. when hair loses protein it splits. pantene helps stop split ends before they start. so your hair passes the needle test. get hair so healthy you shine with pantene. is the most important box in the world.ge, crispy flakes, sons of the sun itself. ♪ the sun'll come out, tomorrow ♪ a magic formula of protein and grain, that helps turn little boys to soccer players. moms to super moms. tomorrow is yours to claim. ♪ the sun'll come out, tomorrow ♪ kellogg's. see you at breakfast™. >> reporter: for three cold colorado weeks, as prosecutors laid out their case for murder, fred mueller's defense attorney roger sagal sat by his side. >> how was fred feeling throughout the trial? >> he was nervous. his life was on the line. there's no doubt about it, it was stressful. >> reporter: fred's children found it especially hard. >> it was the worst experience you can imagine. the worst day of your life, imagine that being analyzed by a roomful of people and drug out publicly and scrutinized. your personal, private tragedy being exposed to the world and picked apart. >> reporter: to the siblings, each day in court felt like an unrelenting attack against their family and their father. >> you can't react. >> you can't say anything. >> you just have to sit there and take it and watch them say all these horrible accusations about my dad. >> you're looking for one person to stand up and say, like this is, this is preposterous. there is no evidence. he's a good guy. he's never shown any history of violence. and, you know, and it was his wife, for goodness sake and can we just, let's, we're stopping this. >> reporter: now, fred's lawyer's were ready to push back. defense co-counsel mike deguerrin told the jurors there was an obvious rush to judgment that contaminated the entire investigation. >> kind of an evil suspicion from the get-go. >> reporter: first-responder michael golob testified that neighbor justin sparks started making accusatory comments while they were still trying to revive leslie. >> justin at one point turned to me and said something along the lines of "i never trusted that sob." >> i think that the suspicions started with mr. sparks and never stopped. >> you think it was somewhat of a snowball effect. >> yeah, i do. >> reporter: and even though golub was a prosecution witness, he admitted he personally didn't find anything odd about fred's behavior that day. >> he just kind of seemed to have trouble focusing. he seemed panicked. everyone deals with tragedy different. i felt in the context it seemed appropriate. >> reporter: but what about that important evidence against fred -- the apparent lack of injuries on leslie's body. >> did you wonder, at all, why your mom didn't have more injuries as -- as some people feel she should? >> i don't -- no. >> no, you can't say what people are supposed to. >> she was bundled up. she was in cold water. we've thought about this more than anyone should ever think about what their body -- mother's body did after she died. >> reporter: the defense called its own forensic expert -- veteran pathologist werner spitz. he testified there were some injuries. bruising on her hand and an apparent small skull fracture that no one else had highlighted. >> if you look carefully at this, you will see that there is an area here that is not like it should be. >> reporter: after looking at the medical examiner's report and leslie's x- rays, dr. spitz said there was no evidence this was a homicide. >> what was it in my opinion? accident. that's what i would have put on the death certificate. >> reporter: a veteran emergency room doctor backed up spitz's conclusion, saying not all falls are catastrophic. >> i don't see how you could rule out a fall from any of the circumstances or injuries that i've seen. >> reporter: and then it was time to counter all those prosecution water tests, the backbone of the state's case. the defense saw major problems with those tests and called its own water expert, a hydrologist who'd been measuring current flows for 40 years. >> increase of stream flow and as a general rule, increase comes in late afternoon and evening. >> reporter: the defense expert pointed out that water in the mountains changes constantly and in 2008, the year leslie died, there was more snow melt than usual. >> was 2008 a high water year? >> it was a tremendously high water year. >> reporter: those prosecution tests were done more than a year later and in a different season -- not spring, but summer -- when the creek's water level is usually lower. >> the flows on august 4th and 5th, 2009 were less than the stream flows on may 3rd and may 4th, 2008. >> reporter: all that so-called evidence that proved fred's story was impossible? the defense said it was useless. >> the testing that you do in 2009 is not necessarily going to be reflective of what did or did not happen in 2008. the bottoms of the creek changes, the walls of the creek can change, there's erosion up creek and you can see it the more times you go up there. you can see, every time it seemed a little bit different. >> reporter: and the defense attacked the state's expert for what it said was another flaw with the mannequin test? >> you did not start that test with the mannequin falling from the ledge, right? >> absolutely not. >> and therefore your testing did not account for any kinetic energy that would have been initiated from the fall, correct? >> that's correct. >> reporter: and what about fred's story? a jumpy dog distracted by a blue jay and a swan dive toward the rocky water? the defense said it was the truth and not at all far-fetched. >> it is steep and angular and there's ice on the rocks it's slippery, so it seemed perfectly plausible that she had fallen and floated. >> reporter: the defense brought in a canine behavior expert who analyzed that last photo of leslie and said the young border collie looked spooked and ready to bolt. >> the dog is under stress, uh, the dog is pushing back. he's pushing back from mrs. mueller. she's trying to control him with the ear pinch and the dog is looking totally off in another direction. >> reporter: to the defense, it was clear -- fred's story was entirely plausible and the prosecution's was not. >> why would he do this? why would a guy who was married 27 years with no history of violence, no history of any sort of domestic disputes, all of a sudden, decide on his vacation that he was going to murder his wife with his bare hands? i mean it just seems, that part of the case seems so farfetched. >> reporter: friend after friend arrived from texas to support fred. even his in-laws took the stand. leslie's own mother, ginnie, vouched for her former son-in-law's character. >> all the years, any signs of fred being abusive or violent to leslie? >> no, i never saw a sign of fred being violent to anyone. >> reporter: and each of fred's children also faced the jury to tell them about their parents' long and happy marriage. ariel described one nightly routine. >> mom and dad would walk to the pond that was in front of our house about 200 yards holding hands and they'd be out there 30 minutes or an hour while the sun would be setting. >> reporter: amanda recalled how devastated her father was in the wake of her mother's death. >> i know he wasn't sleeping well. he would stay in bed a lot during the day. he cried all the time. >> reporter: and alex's testimony brought out an emotional response from fred in court. >> did you ever see your dad able to get over it? >> no, i'm looking at him right now and he's not over it. >> reporter: but what about fred's former assistant, the woman who'd given prosecutors a semblance of motive? in fact, during cross-examination, the assistant said fred and his wife showed affection for each other all the time. >> if he was at his desk, she would come in, she'd go straight to his desk. she'd give him a kiss and she would go back to her desk in the back. >> the absolute bottom line is there was no affair with jeannie barnes and fred did not kill his wife and he certainly did not kill his wife for jeannie barnes. >> reporter: the defense was sure it had shown the jury who fred really was. the people who knew him best said no way in the world could he have killed his wife. but now it was up to 12 people who didn't know fred at all to decide whether he did. coming up, waiting for a verdict. >> the longer it took, the more we were getting nervous. >> and then, a bombshell. >> i was almost in tears. bring in your verizon or at&t bill, turn in your old phone, and we'll cut your rate plan in half. visit us online or visit a sprint store today. thank you cable. for the slower internet upload speeds. for making me wait longer to share my photo albums. thank you cable, because if we never had you... we wouldn't know the incredible difference verizon fios makes. the numbers don't lie. in customer satisfaction studies, fios is rated #1 in internet speed and reliability, and hd picture quality. so join the millions who enjoy the difference fios makes. get a fios triple play online at an amazing price guaranteed for two whole years. plus now get $400 bonus with a two year agreement. call the verizon center for customers with disabilities at 800.974.6006 tty/v >> reporter: after a contentious five week-long trial, the murder case against fred mueller finally went to the jury. >> what was it like waiting for that so important verdict? >> butterflies and tension. >> yeah, we were all so nervous. >> but we were also -- i think we were all pretty confident in our gut that there's no way that he will not be, you know, acquitted. >> reporter: day one of deliberations came and went without a verdict. as the waiting spilled into day two, the defense and prosecution agreed on one thing -- >> that is the toughest time of the trial, is the waiting. you're literally in this holding pattern. >> that's the worst part of any trial. just not knowing what the result was going to be. >> reporter: sheriff bruce was back in his lake city office, waiting by that photo of leslie. >> i -- i had no concerns. we were going through the process and then i'll admit a little -- i questioned why it was taking so long. 'cause i thought we had, it wasn't a slam dunk, but i thought we had a solid case. >> reporter: at the courthouse, the jury was still out. three days, no decision. >> the longer it took the more we were getting nervous. because it seems so clear-cut. >> it seemed obvious to us. >> reporter: it was the fourth day of deliberations when the jurors sent out a note. everyone gathered in the courtroom to hear what they had to say. >> we are at an impasse. >> reporter: the judge offered them lunch and asked for one more try. fred's future was in their hands. >> please rise for the jury. >> reporter: ten minutes later, they'd passed on the free food and were back. >> ladies and gentlemen, i do have another note from you folks and it indicates that we are deadlocked. >> reporter: hopelessly so. the judge had no real choice. he declared a mistrial. >> it was extremely disappointing. >> very disappointing. >> extremely disappointing because it was -- not the -- not the answer that we thought we deserved, that we did deserve. >> mostly i felt for fred, you know, ultimately it's his life that's on trial and so i felt sad for him that we couldn't get this resolved for him in the form of an acquittal. >> reporter: in fact, the defense had been very close -- eleven of the twelve jurors voted "not guilty." >> when i heard the jury vote, i was almost in tears and i thought, "how did they miss -- how did they miss what we were tryin' to say?" >> reporter: after the mistrial, the defense argued for his release on bond and the judge granted it. >> i just, i can't wait to be able to give him a hug and just talk to him whenever i want. >> reporter: now, one full year after losing his freedom, fred was going to get out of jail. his daughters and new wife wendee gathered to embrace him. son alex, who was traveling, called in for an update. >> we're all at the jail right now just waiting for your dad. it's alex. >> we hadn't given him a hug in a year. we haven't been able to see him in person in a year. >> i love you guys. >> it was so wonderful to get the hug that we had waited for. >> you remember what you said to him? >> i love you. i love you. let's go home. when's the next flight out? let's get out of here. come on, let's go home. >> it was so emotional. we were all so excited. we finally got to take dad home. >> reporter: home to texas, but for how long? fred was only out on bond. he still was charged with murder and had the threat of a retrial hanging over his head. >> we were extremely hopeful that they could change their minds and decide not to go forward again. >> and we were really hoping for that. >> reporter: more than just hoping. in fact, the family actually met with prosecutors and lobbied against another trial. >> did that weigh into whether to go to the second trial at all, the fact that this entire family, leslie's family included, did not want a trial? >> absolutely. we have to consider their position. we have to consider where they're coming from. >> reporter: then, one part of the decision was made for them. the judge threw out the most serious charge -- first degree murder. why? the judge ruled prosecutors didn't have enough evidence to establish premeditation. >> we did manage to get an acquittal on the first degree murder charge. >> reporter: prosecutors could still move ahead with second degree murder, but would they? that eleven to one vote surely made a retrial daunting. >> eleven to one is not a great outcome for the prosecution's perspective. >> reporter: but the prosecutors spoke to some of those jurors and found out, to many of them, "not guilty" didn't mean "innocent." >> and kind of behind those numbers were a group of people who by and large felt that something had happened here. that fred mueller was not telling the truth. >> reporter: prosecutors weighed it all and made their decision -- fred would stand trial for murder again. >> i remember having a moment thinking, "i-- i can't do this again." and my mom always would be like, "well, you're going to do it." and i felt like she was like, "and you're going to be fine. and it's going to be good." like -- >> your mom was kind of guiding you through in your head. >> yeah. she was always kind of -- "suck it up. you can do it." >> reporter: but the second trial would by no means be a carbon copy of the first. with new attorneys and new witnesses, both sides hoped that this time they'd see a different result. coming up -- the defense comes out swinging. >> what the prosecution asks you to believe is that fred went from this, to a homicidal maniac within seconds. >> then, a defense witness says something that might do fred more harm than good. >> fred is very successful. he could sell anything. >> would jurors think fred would have sold everyone alive about what happened to leslie, when "dateline" continues. did ya know? 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>> yeah. the dog was right here, and he was just petting the dog. >> she felt that the defendant's demeanor was oddly calm. and that he bent down and petted the dog. she felt like his demeanor was misplaced. >> reporter: for the prosecution, this trial was a second chance to explain to the jury how fred's story didn't match up with the landscape. >> this area was not vast. it wasn't huge. it was actually quite tight, and it was actually quite small. making the defendant's statement impossible. >> our primary goal in the second trial was to get that jury to the scene. >> reporter: the judge considered a visit to cottonwood creek but ruled it was too expensive and impractical to bring the jury that far. >> not being able to do that, we did our best in the second trial to bring the scene to the jury. >> reporter: this second jury had a new 3-d model of cottonwood creek to examine, and fresh testimony to consider. that apparent small skull fracture of leslie's that the defense highlighted in the first trial? no way, said the state's new witness, who was an expert in reading x-rays. >> could you determine that there was any evidence of a fall from 20 feet onto a hard granite surface? >> i didn't see any evidence of that kind of trauma. i didn't see any other fractures. >> reporter: prosecutors hammered home their key points. leslie had no injuries from a fall and tests showed she could not have floated downstream, as fred said she did. >> it just doesn't seem as a likely scenario. >> reporter: so, they argued, fred must have drowned her right where she was found, right after a violent struggle. >> it was our intention to streamline the second trial as much as possible and focus in on the critical points of the case. >> reporter: but fred mueller also changed things up in this trial. he hired a new powerhouse attorney, pamela mackey, who had a roster of high profile clients, like kobe bryant. >> what the prosecution asks you to believe is that fred mueller went from this to a homicidal maniac within seconds. >> reporter: fred's new lawyer argued that the state's case was beyond flimsy. >> their theory that because leslie mueller did not suffer any significant injuries in her fall from the ledge, fred mueller must've drowned her. that's their case. they won't give you the why of that theory. they won't give you the how. >> reporter: the defense again criticized the prosecution's re-creations, and told this jury there was no way anyone could know for sure what the water flow and depth were on the day leslie died. >> if someone measured the actual stream flow of cottonwood creek within days or a week of the incident it would've been informative. >> reporter: and several new defense witnesses took the stand. there was a biomechanical engineer who'd examined the mannequin used in this test. >> well, frankly, i mean, it's not even right to call it testing. >> reporter: according to the defense's new expert, the state's test had a big flaw. that dummy couldn't sink or float in water the same way a human does. in fact, it wasn't even designed for this sort of experiment. >> as far as buoyancy is concerned it is not a scientific instrument for determining that, so it's the wrong tool for scientifically probing. >> reporter: and this time, the defense brought in a memory expert to explain away possible holes in fred's story. the expert said extremely stressful situations can play tricks with memories. so maybe fred didn't actually see leslie fall on her head, as he said he did. >> false memories happen to us all the time. it's not a lie. it's just a memory that happens to be incorrect. >> reporter: but no expert could help the jurors understand how fred truly felt about his wife of almost 27 years. to wrap up their case, the defense brought up fred's family to talk about how happy the muellers had been. >> did you observe anything that led you to believe your folks were having difficulty in their marriage. >> no never. >> reporter: leslie's mother, ginnie, again defended fred. >> i've always had a good relationship with fred. >> reporter: but the prosecution's ears perked up when she offered a new detail about her son-in-law. >> yeah, fred is very successful. he could sell anything. he's a wonderful salesman. >> reporter: in final statements, the prosecutor used those words against fred. >> what we learned about the defendant yesterday is he can sell anything. >> matthew durkin: from our perspective, that's what he tried to do. is that he tried to sell the emergency responders, c.b.i., the sheriff's office and, quite frankly, these juries -- tried to sell something, a story that was just simply impossible. >> reporter: could this jury rise to the challenge and once and for all, make sense of what happened at cottonwood creek? coming up -- jurors get the case. >> either you believe crazy story a or you believe crazy story b, that fred, with a story a or you believe crazy story b, that fred, with a history of violence, killed hi story a or you believe crazy syou fifteen percent or more on huh, fiftcar insurance.uld save yeah, everybody knows that. well, did you know that playing cards with kenny rogers gets old pretty fast? ♪ you got to know when to hold'em. ♪ ♪ know when to fold 'em. ♪ know when to walk away. ♪ know when to run. ♪ you never count your money, ♪ when you're sitting at the ta...♪ what? you get it? i get the gist, yeah. geico. fifteen minutes could save you fifteen percent or more on car insurance. about adding up all the taxes and fees to your monthly bill. that's why our plans start at just $35 bucks, after $5 auto pay credit, all in. taxes and fees included. welcome bigger savings at the sears year end event. use your sears card and get an extra 15% off all appliances. or 36 months special financing on appliances over $499. sears. try new always discreet, a revolution in bladder leak protection from always, the experts in feminine protection. new always discreet, for sensitive bladders. up to 40% thinner, for superior comfort. absorbs 2x more than you may need, for dance-all-you want protection. no wonder more women already prefer new always discreet pads over poise. new always discreet. now bladder leaks can feel like no big deal. because, hey, pee happens. reporter: it took more than five years, dozens of witnesses and hundreds of pieces of evidence in two colorado courtrooms to get to this moment. again, twelve men and women were trying to decide if leslie mueller was murdered or a victim of a freak accident. >> we all agree that the accident, the fall, was completely crazy. >> and i just kept thinking, well, crazy things happen in life all the time. >> we spoke to 7 of the 12 jurors. >> either you believe crazy story "a", that she fell off that cliff and down three waterfalls, or you believe crazy story "b", that fred who has no history of violence, killed his wife. both stories are crazy. which one do you want to go with? >> reporter: what were the key pieces of evidence for you or the lack thereof? >> lack of injuries. >> reporter: you felt that she should've had more injuries? >> absolutely. i mean, to fall 15 to 20 feet onto a granite platform. >> face first. >> and to go down head-first. >> down three waterfalls. and be pulled out and described as pristine? >> reporter: show of hands of who believed fred's story. >> at one point and time, we all said -- >> i kind of -- >> all right, let's vote and see who -- >> where we're at. >> -- who thinks he's lying. and we all raise hands. okay, we convicted him of lying. >> the question beyond that was my question, is, if we don't believe his story, does that make him guilty? he is not on trial for lying. he's on trial for murder. and i need to know how and why. >> reporter: the jurors wondered where was the evidence that fred drowned her? if he really did kill her, prosecutors never provided any clear explanation of how he did it? >> there was no evidence to put him down at the recovery site. there were no footprints. and there was no defensive wounds on her. if there was a scuffle, i would think she would have -- >> she would have fought back. >> some bruises. she would have fought back. >> reporter: they poured over technical testimony about water and the creek - - to them that flood of data and expert testimony was a complete wash. >> you can bring all the experts in and all the water tests, and it was just pointless because they'll always poke a hole in it because it wasn't done may 3rd, 2008. >> reporter: like the jury before them, this group's first day of deliberations ended without a verdict. >> as the deliberations took longer and longer, it became clear that we needed to be nervous again. >> reporter: while sheriff bruce felt more confident than ever this time around. >> i slept well. i ate well. i went about the rest of my normal duties at work here without a whole lot of concern about it, and just expected that phone call to come in and say, "we got him." >> reporter: the 12 jurors returned the next morning, determined to make a fresh start. was anyone here consistently thinking about leslie, the mother, the wife, the doctor? >> we had little pictures up and everything. and i made the comment, like, "leslie, talk to me, say something." >> 'cause she can't speak, she was -- >> she needed us. >> reporter: after two days of intense debate, the 12 jurors thought they were done. they sent out a note and the judge called everyone into the courtroom. >> ladies and gentlemen, you have sent me a final note, indicating that you do not believe that you can reach a verdict. mr. foreperson, is that correct? >> that is correct, your honor. >> okay. i am gonna declare the jury deadlocked in this case. >> reporter: hopelessly deadlocked/ a second jury unable to say for sure if fred mueller had killed his wife or not. when you hear those two words again, "hung jury" -- >> it's crushing, crushing. >> absolutely crushing. >> reporter: disappointment again on both sides. do you ever look back and think, "you know what? maybe we shouldn't have done this? we just, it wasn't there from the beginning." >> absolutely not. >> no. >> both these trials were worthy of prosecution. a woman was murdered and both the efforts were more than worth it. >> reporter: so how close was the jury's vote this time? eight jurors believed fred was guilty and four voted to acquit. >> i have too many questions that are unanswered. therefore, i have reasonable doubt. >> we just couldn't come to agreement, you know. i was steadfast guilty and he was steadfast not guilty. and we tried to get everybody's impression of what happened or what they thought. >> reporter: the prosecutors wondered, could one decision by the judge have changed it all? >> to this day i believe that had either one of these juries been able to go to that scene, perhaps it would have been a different result. >> reporter: and then, three weeks after the second mistrial, an announcement, the state dropped all charges against fred mueller. >> when we finally got the news, it was, like, overwhelming with joy. we finally can rest at ease. and my dad can move on with his life. and we can finally just move on and -- >> relax! >> reporter: fred and his wife wendee have returned to texas, where fred still runs his thriving business. in colorado, the case officially remains open. >> without any further investigation, without any further witnesses that could come forward to provide, some new bit of evidence, at this point the case is -- just simply remains open. >> reporter: so fred will essentially have this hanging over his head potentially forever? >> at this point the case remains open. fred told us he's well aware of this legal limbo. he said he wanted to do an interview with dateline but is concerned that anything he says could be distorted by the prosecutors and investigators who still believe he did something wrong. >> leslie got to me. >> reporter: and that picture of leslie is still prominently displayed in sheriff ron bruce's office. he says even when he retires, that photo goes with him. >> i think it was just the magnitude of the terrible tragedy that stuck with me, and will stay with me the rest of my life. >> reporter: you get teary-eyed when you talk about this woman you've never even met. >> correct. >> reporter: who's not even a member of your family. >> yeah, i do. >> reporter: what is this connection you feel with her? >> i don't have an explanation for this. i can't put a finger on it. but certainly, it does truly haunt me, yeah. i feel like i let her down. >> reporter: leslie's family chooses to remember her, vividly, not as a victim but as a cherished wife and mother they lost too soon. >> we don't want people to forget about how wonderful of a person she was before the accident and all the great things that she did. >> all the babies she delivered and -- >> lives she saved. >> a great, great person. >> reporter: with the vacation home the muellers once loved long sold, they are making new memories as a family far from colorado. and in those magnificent rockies? seasons change. waters rise and fall. and some believe the secrets of cottonwood creek will remain a mystery forever. >> that's all for this edition of "dateline." we'll see you again next friday at 9:00, 8:00 central. i'm lester holt. for all of us at nbc news, good >> announcer: nbc 10 news starts now. >> right now, at 11:00, a truck crashes into a home. >> all of the sudden, there was a vibration, all houses shook. >> hours later, a woman dies on her 93rd birthday. police track down the selected hit and run driver. tonight, why he's no longer in police custody. good evening. the bucks county district attorney will decide next week how to charge that driver who they say took off after crashing into the elderly woman's bedroom in levittown. she was a hospice patient and died only hours later on her birthday.

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