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> reporter: if your travels take you to the ozarks in southwestern missouri, that splendi">
"dateline." here's dennis murphy with "the collector." >> reporter: if your travels take you to the ozarks in southwestern missouri, that splendid lake country, you maybe have bass or trout fishing on the mind. or perhaps you're heading to branson to catch a few of the stage shows. but there was no tour guide that would have directed you to this modest home in springfield, missouri, and that's a shame because it was an old curiosity shop well worth seeing. floor to ceiling collectibles. valuable stuff everywhere the eye landed. but still every object just so. think neat hoarders. >> anything from coins to arrowheads to -- he had probably 10,000 to 15,000 books. >> he's big into gold and silver, gems. >> bayonets from world war i and just, all kinds of stuff that he, you know, a leg cannon. >> reporter: but did these collectors who had such a knack for finding treasure have a less discerning eye when it came to sizing up people? did that go towards explaining the dreadful toll in that house that evening? >> i said, he's off in some way. i don't know. he could be a psychopath or something. i said it scares me. >> reporter: on april 30, 2014, did they fall prey to someone they believed could be trusted? >> i had to be lowered to the ground. and i started crying and screaming. >> oh, not those -- oh, such good people. why? >> reporter: springfield, missouri, is called the queen of the ozarks, a church-going, neighborly city of midsize with good colleges. gary tyrrell grew up poor on a farm not far away and knew early on that his adult life would be all about education, teaching kids in his classrooms by day, the history book in his lap at night. he and his wife jan raised two kids, a girl and a boy who sadly died young of a brain cancer. jan kept the house mainly and sold avon products part time. they lived and gardened in the same house for almost 25 years. they even designed it themselves. life in missouri was good for the two. >> they loved each other very much. >> reporter: daughter jessica remembers how extraordinarily close her parents were, joined at the hip. >> if my mom went shopping, my dad took her. they gardened together. talked on the phone, you know, several times a day if they were apart. they really did everything. >> reporter: so they really were for the ages, huh? >> yes, yes. >> reporter: personalities? gary was the fun, outgoing one. >> he was a very jolly man. you know, kind of like a santa claus. >> reporter: jan, a little more reserved. >> it took a little bit more to get to know her. and then once she warmed up to you, she was very alive. >> reporter: when gary retired as a beloved assistant school superintendent, he suddenly had all the time in the world to visit civil war battlefields, scour flea markets and pore over his antique dealers catalogs. but there was a lucrative sideline to his obsession for memorabilia. >> and he was big into buying and selling gold and silver. >> reporter: gary's brother, larry tyrrell. >> he constantly collected coins and bought gold and silver trinkets. >> reporter: and did pretty well at it? >> he did very well at it. gary was a good businessman. >> reporter: the gold and silver went into one of five safes in the house. but the coins and artifacts, many of them museum quality, were out on display. >> he had a lot of rare pieces. a lot of indian jewelry that was very quality pieces that he loved. >> reporter: what was his most unusual piece, do you think? >> probably be the walrus tusks that he owned. he had three of those. >> reporter: they were rare 19th century hand engraved scrimshaw. and each worth around $10,000. so how would he find these things? >> i'm not really sure. they did go to flea markets and garage sales and things like that, but any time i would ask him, you know, where did you get this? he would say, oh, in my travels. >> reporter: in 2013, jason murray, became one of the family when he and jessica got engaged. he remembers the walrus tusks as well as all the other unusual items in the tyrrell home. >> it was one of those things where like there was so much stuff in the house and yet it was immaculate. everything had a place. and he had so much stuff, it was like an offbeat museum. >> reporter: jessica and jason lived almost 300 miles away in oklahoma city. but jessica spoke with or texted her parents several times a day. but that all changed on may 1st. >> i had called my mom on my way to work, like i always do. and she didn't answer. so i thought, well, maybe she's in the bathroom. maybe she's outside. and i called back, and i still didn't reach her. >> reporter: by noon, jessica says she still couldn't reach her. so she called the springfield police and asked them to go by the house. and when the officer did that, what did he report back to you? >> that nothing seemed out of place. they couldn't see anything inside the house. all the doors and -- were secure. absolutely no signs of forced entry or that anything appeared to be out of place. >> reporter: the officer had no cause to force his way into the house and left. but jessica says she was still worried. so she and jason jumped in their car to make the five-hour drive to springfield. they arrived at the house around 7:00 p.m. >> as we were coming around the corner to the house, i hit the garage door button. and both of their cars were in the garage. >> reporter: and that was bad news for you? >> yes. >> reporter: what did that imply? >> that there was something terribly wrong. >> reporter: jessica called 911, without ever going past the garage into the house. two officers arrived within minutes. >> they entered the home. and then another officer arrived and then another and then another and then another. >> reporter: and you're waiting outside? >> and we're waiting outside. >> reporter: no one said a word to jessica and jason. and then, they noticed police blocking off the area with crime scene tape. >> i kind of grabbed on to the officer and i said, what is going on? why are they calling for crime scene tape? and he kind of, you know, held me and said, i don't know what's going on. all i know is there are two people in the house that are deceased. chilling words outside the tyrrells' house. but they were nothing compared to what investigators would find inside. when we come back -- >> it seems to be an element of personal anger, this close quarter killing of somebody with a weapon in your hand. >> it definitely seemed to be personal. the brutality involved. now, wes of clothes cleaned in one wash. has anybody seen my pants? i found em ellen! put those on, dad! lasts up to two times longer! tide, number one rated. when you have type 2 diabetes, like me, hi, i'm dominique wilkins. there's a moment of truth. and with victoza®, a better moment of proof. victoza® lowers my a1c and blood sugar better than the leading branded pill, which didn't get me to my goal. victoza® works with your body to lower blood sugar in three ways-- in the stomach, the liver and the pancreas. and while it isn't for weight loss, victoza® may help you lose some weight. non-insulin victoza® comes in a pen and is taken once a day. (announcer) victoza® is not recommended as the first medication to treat diabetes and is not for people with type 1 diabetes or diabetic ketoacidosis. do not take victoza® if you have a personal or family history of medullary thyroid cancer, multiple endocrine neoplasia syndrome type 2, or if you are allergic to victoza® or any of its ingredients. stop taking victoza® and call your doctor right away if you get a lump or swelling in your neck or if you develop any allergic symptoms including itching, rash, or difficulty breathing. serious side effects may happen, including pancreatitis. so, stop taking victoza® and call your doctor right away if you have severe pain in your stomach area. tell your doctor your medical history. taking victoza® with a sulfonylurea or insulin may cause low blood sugar. the most common side effects are headache, nausea, diarrhea, and vomiting. side effects can lead to dehydration, which may cause kidney problems. now's the time for a better moment of proof. ask your doctor about victoza®. as darkness closed in on springfield, missouri, the daughter who'd asked local police to check on her parents, was now being told two bodies were found inside her parents' home. they didn't identify them, but jessica says she knew it could only be her mom and dad. >> and i started crying and screaming because i knew it was -- it was my worst fear. >> reporter: her fiance, jason, was by her side, trying to console her. >> she was beyond upset. she kept saying, i'm sorry, mom. i'm sorry mom. >> i had no details. i had no idea what happened. >> i'm thinking this is horrible and also hoping it's carbon monoxide. >> reporter: not benign but something explainable. >> right, right. >> reporter: but inside officers quickly grasped the cold stomach-churning truth. jan and gary's deaths were not accidental. >> it was going to be a long investigation. >> reporter: detective neil mcamuss was one of the first detectives on the scene. that night he entered the home through the garage and saw gary first. >> we could see that there was a deceased male in the hallway of the home. >> reporter: his pants were pulled down. what did you make of that? >> we didn't know. the only thing that we could think of was he's trying to get away from somebody, that they're grabbing hold of him. >> reporter: or was the killer rifling his pockets for something? a key, maybe? a combination? in gary's office downstairs, the detective saw jan. >> she was on the ground. she was lying face down. but it was obviously that there was severe trauma to the back of her head. >> reporter: so was this a violent home invasion? jan and gary had more than a half a million dollars worth of valuables in the house there for the grabbing, so a botched robbery was a likely scenario. and in the garage, police did find strange marks and scratches on a tornado shelter that was also used as a safe. >> it appeared that somebody had tried to pry the lock open. there were some marks that were by the lock on the door of the tornado shelter. >> reporter: they didn't get inside. >> it did not appear that they did, no. >> reporter: but that would suggest somebody's looking for something. >> yes. >> reporter: but if robbery was the plan, the killer had left the oddball museum strangely intact. nothing appeared tossed or out of place. and the nature of the killings -- gary was shot as well as beaten -- spoke volumes to the prosecutor assigned to the case, todd myers. it seems to be an element of personal anger in this thing, todd. this close quarter killing of somebody with a weapon in your hand. >> yes, it definitely seemed to be a personal-type killing, just the brutality involved. >> reporter: investigators determined jan and gary were likely killed the night before. but search as they might for the murder weapons, the gun and the bludgeoning instrument, officers came up empty handed. but they did recover some unusual evidence. >> around jan's body there were small white flakes of a substance that were obviously either from the murder weapon or on the murderer's body. there were similar flakes found by the wound to gary's head. >> reporter: so they bagged them and sent them off for testing. and the crime scene techs recovered something else that could be a crucial piece of evidence, a discarded latex glove. so a latex glove on the floor jumped out at you. >> totally out of place. and from talking with everybody, that there is no way that gary or jan would have left that glove there. it was right in the middle of the floor for everybody to see. >> reporter: did it belong to the victims or the killer? no one knew. so it's collected. you don't know what it means. >> we don't know what it means. and it was collected. sent to the highway patrol crime lab. >> reporter: they had no idea, no working theory yet of why the couple had been murdered. but they did have a sequence that made sense as to the order of deaths. >> it is hard to believe that gary is shot upstairs if jan is still fully functioning and mobile that she would not have called 911, would not have tried to intervene in some way. >> speculation but common sense tells you she's the first victim. >> she's the first victim. yes. >> reporter: she's bludgeoned to death downstairs, gary arrives at some point later. >> most likely he arrives either while that is ongoing or the killer is waiting for them. but yes, it appears that she would have been the first to have died. >> reporter: a big question early on for detectives was understanding their crime scene. how did the killer gain entry to the house? >> there were no signs of forced entry. >> reporter: windows haven't been forced. the doors are all intact. how did the killer get in? >> there are only two ways the killer could have gotten in. one would be to have their own access device, whether it's a key, a garage door opener, some way that would open the doors up. and then upon leaving, be able to lock it that way. or, to have been let into the house by either gary or jan. >> reporter: it didn't appear to the cops to be a random home invasion but rather that the killer or killers had been someone the couple knew. was it a person gary encountered in his antiquing travels? or, terrible to think about it, was the perpetrator someone much closer to home? was this all in the family? >> i was shocked. really confused and baffled. coming up -- who had a motive to kill the tyrrells? investigators have at least one idea. >> she's the person who could benefit here from this crime in terms of getting money. >> she's the only child from parents that are very well-to-do with a lot that's going to be left to her. >> when "dateline" continues. create your own tour of italy is back starting at $12.99, only at olive garden. choose 3 of 9 of our favorite italian dishes to get everything you want, all on one plate. plus unlimited salad and breadsticks. because the best tour of italy is the one you create. olive garden. tthe whitenessmy wasn't there as much, my teeth didn't look as healthy as others. my dentist said that pronamel would help protect my teeth. pronamel is giving me the confidence to know that i'm doing the right thing so it's nice to know that it was as simple as that. ♪olympics 2016, let me get you on my level. ♪ so you never miss a moment, ♪ ♪miss a minute, miss a medal. ♪ ♪ why settle when you can have it all? 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>> yes. >> reporter: so that night, detectives asked jessica and fiance jason down to the station to answer a few questions. the cops didn't let on they were talking to them not just as grieving relatives but also as potential suspects. >> i'm obviously going to take a few notes, obviously, as we're talking. so don't let that bother you. >> reporter: the interviewer withheld details about the crime scene. a standard investigative tactic to see if a subject knows more than they should. he even threw out a theory that wasn't true -- murder/suicide. >> one of the things we have to look at is as to whether or not that one of them would've injured the other and then maybe themselves. >> reporter: jessica seemed to go with it. >> do you think that's a possibility? >> my dad, since his mom died, has been super depressed and has made comments. and my mom called the doctor's office, and i told her if he did -- if he said something stupid before -- again, that she needed to call the police. like get the [ bleep ] away from him and call the police. >> reporter: and as they continued talking, police had another reason to be suspicious of the daughter. she'd driven five hours to check on her parents then didn't go inside the house. wouldn't you wonder why she would do that? >> yes. people sometimes that commit a crime or know a crime that's been committed, you know, they don't want to see the bodies in there so they call somebody else. >> reporter: detectives pressed the couple about their movements that day, what they did or did not do at the scene. had they tampered with that tornado shelter in the garage. >> you guys didn't do anything to try to get inside? >> oh, no, no, no, no. i mean i pushed my hand on the handle, it was locked and that was it. >> reporter: did you think that maybe are we looking at a situation here where we have to call a lawyer or be careful about what we say? >> i did, a little bit. >> reporter: but neither jason nor jessica asked for a lawyer and offered the detective more detail about what was in that tornado shelter that had been tampered with. >> the safe room was full of gold, and silver, and gold bars and i don't even know -- >> we actually don't know what all's in there. >> i mean he has books that are signed by presidents. he has indian peace medals. >> reporter: for investigators, the entire interview was a test. >> how are you going to hold up? are you going to give answers that we can go back and verify? >> reporter: you so intend to sweat them? >> yes that was the detective's intent to the degree of just trying to make sure we get the truth. >> reporter: did they pass the tests? detectives let them go but held on to jessica's car to test it for potential evidence. why did they have reason to look in your car? >> i didn't really know. i assumed it was just because my car was there. >> reporter: now jessica had to call family and friends to tell them the news about her parents. her uncle larry, her father's brother, was at his law office when he got the call. >> she said mom and dad are gone. i said, what do you mean? are they out of town? no, they're deceased. i collapsed on the floor. >> reporter: you were a mess. >> i was a mess. the unknown was terrifying. because i didn't know what had happened. >> reporter: but larry wasn't shocked or outraged to learn his niece was being looked at as a potential suspect. he was, after all, a criminal defense attorney and former prosecutor. >> i knew she had to be eliminated. i understood that. due to the fact that she was the only child. she just had to be eliminated so that the case could move on. >> reporter: and the case did keep moving. but after a full week of investigating the daughter and fiance were not eliminated. rather, detectives called them back to the station, this time interviewed separately. jason first. cops by now had figured out that the murders took place the night before the bodies were discovered. so where exactly were he and jessica then? >> we went to the pizza hut up on -- i don't know, probably 5:30 or something like that and came back home and stayed home. >> reporter: home five hours away in oklahoma city. >> you remember how you paid for the pizza hut? >> on my credit card. >> reporter: then right to the point. >> you have any reason to want to harm gary or jan? did go into the house. everything you telling me the truth? >> absolutely. >> reporter: next jessica was in the chair answering questions about her relationship with her parents. >> are they paying any of your bills? >> they pay my at&t bill for my cell phone. pay my car insurance. and i think that's it. >> reporter: i'm just going to ask. did you have any ill feelings towards your folks? anything that you would have been mad at them for, any reason to want to harm them or anything like that? >> no. my parents were my life. >> reporter: so better your financial situation or anything like that? >> no. i mean, if i needed something, all i had to do was call and ask. >> reporter: so you're a suspect. >> right. >> reporter: in your parents' double homicide? >> yes. >> reporter: how do you deal with that? you haven't even buried them yet. >> i wasn't really dealing with it. it was more just going through and saying i -- you know, i didn't do it. >> reporter: police asked both jessica and jason for dna samples, then set out to verify their alibis. >> my boss and i took a trip to oklahoma city, and we went to employers, spoke with fellow employees, with jessica's boss, and we went to the pizza hut to see if we could see some surveillance video. >> reporter: would it all check out or would an unexpected lead send the investigation in a new direction? this was a huge bingo moment. >> it was. coming up -- gary's best friend offers detectives some clues including texts he says he saw on gary's phone. >> there were five in a row from a guy that he had taught in a class asking for money. >> did someone need cash badly enough to kill? before fibromyalgia, i kept on top of things. i was energetic. then the chronic, widespread pain drained my energy. my doctor said moving more helps ease fibromyalgia pain. he also prescribed lyrica. fibromyalgia is thought to be the result of overactive nerves. lyrica is believed to calm these nerves. for some, lyrica can significantly relieve fibromyalgia pain and improve function, so i feel better. lyrica may cause serious allergic reactions or suicidal thoughts or actions. tell your doctor right away if you have these, new or worsening depression, or unusual changes in mood or behavior. or swelling, trouble breathing, rash, hives, blisters, muscle pain with fever, tired feeling, or blurry vision. common side effects are dizziness, sleepiness, weight gain and swelling of hands, legs and feet. don't drink alcohol while taking lyrica. don't drive or use machinery until you know how lyrica affects you. those who have had a drug or alcohol problem may be more likely to misuse lyrica. with less pain, i can be more active. ask your doctor about lyrica. want great whitening without the mess? 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>> reporter: you might not have believed it or your circle of people didn't believe it or knew -- >> my circle -- my circle was not believing that. >> reporter: and behind the scenes, police and prosecutors had come to the same conclusion -- jessica and jason were innocent. >> everything that jessica and jason were telling the police, the police were able to corroborate and verify. >> and they went through everything. they went through the phone records. the alibi was checked out top to bottom. >> it was. >> reporter: and when the dna results came back on the latex glove, neither of them was a match. the dna belonged to an unknown male. was that a relief for you? >> yes, i mean, we knew we didn't do it, so, i mean, it was like okay now you need to find who did it. >> reporter: the couple felt they could help their investigation with their own amateur sleuthing. for instance when jason went back to the house for the first time, he noticed something was missing. >> there was a display of little ivory pieces and multiple walrus tusks that are engraved art type things. they had been moved. >> reporter: walrus tusk, are we talking about like so? >> the big one was about yea big. the larger one wasn't where it was supposed to be. and the other wasn't there at all. >> reporter: jason told police that one of gary's prized tusks could be the murder weapon. the source of those mysterious white flakes. jessica noticed someone had been at her dad's desk where he sorted coins. >> he kept them around his desk in coffee cans and bags, and he would be sorting them or organizing them. >> reporter: and a container with what would have looked like loose parking meter change was missing. its actual value around $20,000 according to her dad's records. the killer would have known the real value only if he moved in the same antique circles as gary, at least that's what the brother suspected. >> perhaps gary went to buy some gold or silver and someone had set him up and had someone follow him home and got the drop on him or jan. and had robbed him and killed him. >> reporter: sounds very plausible. >> that was a big worry i had. >> reporter: but as they dug into gary's business dealings, no potential suspects emerged. few people had any idea how wealthy they really were. >> they did not show that type of wealth to people. it was always kept very quiet. the people that he would interact with, as far as selling the gold, were always very reputable type people. >> reporter: and because there was no forced entry to the home, larry offered up the very limited list of people who had access to the house. a handful of local contractors. i assume you were asking the garden guy and the cabinetmaker, whoever might have been in the house from larry's list of people to give dna? >> right. >> reporter: at the same time officers were reaching out to others in the couple's inner circle. gary's closest friend was mark porter, a married father of two and a prominent guy around town and a former school superintendent. >> for us it's just a wise thing to do. >> he was handsome. he looked good. he dressed good. when he first came to the district he drove a jaguar. >> reporter: nancy worked with both gary and mark. >> and they seemed to get along really well. worked well together. i thought they were friends at school and off of school. >> reporter: so mark went down do the police station to talk with investigators. >> all right. mr. porter. it's mark, correct? >> yeah. >> do you mind if i call you mark? >> no. >> i just want to kind of touch base with you and talk about how you know them, how long you've known them. >> i've known gary since probably maybe 15, 20 years. he was my assistant superintendent. i was the superintendent. >> reporter: mark knew a lot about gary's coin business. he had started dabbling in it as well. he told detectives details that suggested a fellow collector could have targeted the tyrrells. >> he's got people all over missouri that collect for him. and i know he goes to hotels and meets people and he runs ads in the paper. >> reporter: according to mark gary set up one of those meetings around the time of his death and it involved a lot of money. >> he was actually going to sell $50,000 of gold for cash. it was actually a buyer who was going to give him $50,000 in cash, that's all i know. >> reporter: he also mentioned text messages he'd seen on gary's phone from some other guy. >> gary asked me to read him some of the text messages. there were five in a row from a guy that he taught in a class that was asking for money. >> okay. >> and i have no idea -- >> you don't remember, was there a name attached to the text? or a number? >> i can't remember. >> reporter: and remember, gary was shot as well as beaten. could it have been with one of his own guns? >> gary has guns everywhere. gary has guns in every drawer of the house, i think. i mean, he likes his guns. he loves guns. >> reporter: but then mark gave detectives a detail that took them completely by surprise. >> he was supposed to meet me, actually, at 8:00 on wednesday, and didn't show. >> reporter: wednesday, the night of the murders. it was the first police were hearing of gary having plans that night. what else did the best friend know? coming up -- mark gives a detailed account, one that troubles investigators. >> there were several things about that story that raised some questions. >> and then, larry's brother reveals who he thinks had a motive for murder. >> i know he had some sort of bad financial issue. with 100% nutrition, 0% fillers, always real meat #1. lifelong smart nutrition. it's all in one. purina one. if you a dry mouth can be ations common side effect. that's why there's biotene. it comes in oral rinse, spray or gel, so there's moisturizing relief for everyone. biotene. for people who suffer from a dry mouth. a leading consumer testing the top laundry detergents. the winner - persil 2 in 1, didn't only beat tide... it beat every single detergent tested. boom. switch to persil proclean 2 in 1. #1 rated. is back starting at $12.99, only at olive garden.tour of italy choose 3 of 9 of our favorite italian dishes to get everything you want, all on one plate. plus unlimited salad and breadsticks. because the best tour of italy is the one you create. olive garden. hi. hey. you're that guy from verizon who switched to sprint. i used to ask if you could hear me now, but it's 2016 ...and sprint's network reliability is now within 1% of verizon. and sprint saves you 50% on most verizon, at&t and t-mobile rates. that's why millions have switched. make that millions plus one. can you hear that? don't let a 1% difference cost you twice as much. switch to sprint today. springfield police were talking to gary's best friend mark porter who suggested plenty of theories about why gary was killed. he said gary's money was cat nip for all kinds of folks. >> people knew they had money were always asking for it. >> reporter: then mark revealed something detectives didn't know. he and gary had plans to meet at mcdonald's on the night of the murders. >> how long do you think you waited on him? >> i waited probably a good -- i got there probably 7:45, probably 45 minutes. >> okay. >> reporter: so the friend said he drove across town to stop by gary's house. >> i went back to the house, knocked on the door. nothing. didn't answer. so i thought maybe they were gone or left. because he'll leave. he'll go and say, oh, man, i forgot, my bad. i'm in kansas city. so i had no clue. >> reporter: mark told detectives that gary had stood him up before. no big deal. so he went back to his office at ibm to do some work. >> now there were several things about that story that raised some questions. >> reporter: like? >> like why would you wait 45 minutes for someone and not pick up your phone and give them a call and say, hey -- >> reporter: what's up? >> what's up? did you forget? >> reporter: but what seemed really strange was that mark porter had just admitted to going to the house the night of the murders with a story that just didn't ring true. why drive across town if he wasn't all that concerned about his friend missing dinner? >> so he drives a few miles across town to go knock on the door when his office is across the street, when it doesn't appear that gary is showing up. >> reporter: so he puts himself at the house, sort of a benign explanation. >> yes. >> reporter: inside the interview room the detectives decided it was time for a break. >> give me a couple seconds. i'll go back there and see if there's anything else. can i get you anything, coke are water? >> reporter: trying to make sense of it all including this bit of information. gary's brother had told police that not long ago mark asked gary for a loan, put the request in writing, not just a little bit till payday but an eye popping $250,000. >> that's the kind of money if you can get it out of your house as a second mortgage maybe you're lucky. >> right. >> reporter: what did your father make of it? did he consider it even for a heartbeat? >> no, not at all. when he was talking to us about it, he was kind of laughing. he said, you know, i don't know where he thinks i would have this kind of money. >> reporter: strange since not only would mark, gary's former boss, have had a nice pension as a former school superintendent, he was currently earning a six-figure salary at ibm. brother larry had suggested mark might have a gambling problem. >> i knew he had some sort of bad financial issue. >> reporter: when the detective came back to the room, the tone of the interview had changed completely. >> have you ever had to ask gary for money yourself? >> no, only for -- only in trade or buying. >> reporter: suddenly mark porter was in the hot seat. >> so if i was getting information from somebody who told me that maybe you had a large gambling debt. >> i do not have a large gambling debt. >> that you asked gary for maybe some assistance. >> no. >> would that be false? >> that's not true. >> reporter: he denied asking for the loan, said the only time he discussed large sums of money with gary was about a real estate deal. >> because he's always looking for investment, whatever. >> reporter: then detectives asked him the question they've asked everyone else. would he offer up a dna sample. >> i have like a cheek swab that i'll ask if you are fine i'll do that today. >> no, i'm going to wait. >> any particular reason? >> i don't know. i don't know the legalities. i don't want to do something then my attorneys say i shouldn't have done that. >> why would you not give us a dna sample? >> reporter: it's your best friend. >> your best friend. we're doing everything we can to find who brutally murdered your best friend and you're telling us that you're not sure you're going to give us a dna sample. >> reporter: the refusal wasn't evidence of mark's guilt, so police let him go, but the investigation suddenly shifted away from the workers at the tyrrell home and toward mark porter. some detectives hit the pavement to check out his alibi while others continued mining over avenues for leads. one cop thought to check a local database of pawn shops to see if any of the missing items from the house had been recently sold. that search them them here to r & k coins. tell us how your business ended up being very important in a murder investigation in spring field. >> yes, sir. >> reporter: it was two days after jan and gary were killed when teddy ellington was working behind his desk and a guy with a tub of coins walked in. >> reporter: what were you seeing in him? was he okay? >> he looked like any average guy to me. nothing actually stood out about him at all. >> reporter: he had piles of dimes, quarters, 50 cent pieces. when you added up the face value of that stack of money, what was it worth? >> he had $1210 in face money. >> reporter: but the meltdown value of the metal itself was can worth what? >> i paid him $18,351. >> reporter: $18,351. a lot of money. and not far off the value of the coins jessica reported missing from her parents' home. >> when he walked in and stacked his coins, your security cameras caught it all? >> sure did. >> reporter: so who was it? coming up -- how a mcdonald's coffee cup was about to blow the case wide open. >> did your dashboard just light up when you hear that? >> can't describe the emotion of it. >> finally a family learning the truth. >> i remember falling to the floor. tresiba® provides powerful a1c reduction. releases slow and steady. works like your body's insulin. when my schedule changes... i want something that delivers. ♪ tresiba® ready ♪ i can take tresiba® any time of day. so if i miss or delay a dose, i take it when i remember, as long as there's at least 8 hours between doses. once in use, it lasts 8 weeks without refrigeration... twice as long as lantus®, which lasts 4 weeks. tresiba® is used to control high blood sugar in adults with diabetes. don't use tresiba® to treat diabetic ketoacidosis, during episodes of low blood sugar, or if you are allergic to any of its ingredients. don't share needles or insulin pens. don't reuse needles. the most common side effect is low blood sugar, which may cause dizziness, sweating, confusion, and headache. check your blood sugar. low blood sugar can be serious and may be life-threatening. injection site reactions may occur. tell your doctor about all medicines you take and all your medical conditions. taking tzds with insulins, like tresiba®, may cause serious side effects like heart failure. your insulin dose shouldn't be changed without asking your doctor. get medical help right away if you have trouble breathing... fast heartbeat, extreme drowsiness, swelling of your face, tongue, or throat, dizziness, or confusion. ask your doctor if you're tresiba® ready. ♪ tresiba® ready ♪ unlike ordinary diapers with two layers, pampers have three pampers. absorbent layers to stay up to three times drier, so babies can sleep soundly all night. wishing you love, sleep and play. pampers. detectives finally had a major break in the investigation. silver coins worth around the same amount as the ones taken from the tyrrell home had turned up in a local coin shop. surveillance cameras caught the transaction on video. the man certainly looked familiar but if there was any doubt he left a copy of his i.d. >> mark porter. >> yes. >> reporter: mark edward porter. this is a real license. this is the real guy. >> that's him. >> reporter: but selling a load of coins didn't prove mark porter was guilty of murder. remember mark also had an interest in the coin business. >> the best explanation for that was just simply that these were coins that mark porter had, it was just a coincidence that they happened to be sold the day after the tyrrells were killed. >> reporter: still for police, the friend, ibm executive, retired school superintendent and by all appearances a pillar of the community, was now a prime suspect in the murders of jan and gary tyrrell. >> i never did like the guy. i never did trust him. i thought he was something of a fake. >> reporter: gary's brother larry always had a bad feeling about mark. and he was convinced if mark was the killer, the motive was money. >> i felt that he was befriending and recruiting and grooming my brother to gain access to his money. i think over a period of time he saw jan as an obstacle. >> reporter: when jessica heard detectives were looking at mark, she wasn't surprised. >> he didn't show up at the funeral. >> reporter: conspicuous in his absence? or am i making too much of his nonappearance? >> i think it was pretty obvious that there was something going on. >> reporter: and his former secretary said she hated working for mark porter. ended up quitting her job to get away from him. >> i said he's off in some way. i don't know. he could be a psychopath or something, i said, it scares me. >> reporter: you used that word? >> i did. >> reporter: but what bothered police was his lack of cooperation. >> he wouldn't give dna. he wouldn't take a polygraph. >> reporter: add to that the suspicious pawn shop transaction and the story about stopping by the tyrrell house on the night of the murders and a talk with mark's wife about the night of the killings only made them more certain. >> she had talked about making a couple phone calls to mark, and that she couldn't get hold of him and she didn't know where he was or what he was doing. >> reporter: but police still needed physical evidence, a dna sample to match to that latex glove found at the scene. so they hatched a plan to get one. you start tailing mark porter? >> yes. >> reporter: cars outside the house? >> yes, we had undercover officers that were constantly doing surveillance for weeks and weeks and weeks to try and somehow obtain a dna sample from him. >> reporter: but nabbing a sample proved more difficult than expected. in all those weeks, officers continued to come up empty. >> they watched him eat his lunch. they watched him get a to-go cup. and they were all excited because they thought they were going to be able to get the straw he'd been drinking his drink with only to see him pick the straw up and put it into his to-go cup and leave with the straw. >> reporter: so he knew that you guys had eyes on him? >> it's difficult to know what he knew at that time. >> reporter: then three months after the murders, an undercover officer followed mark into an automobile oil change shop. >> goes inside, mark's got a coffee cup that he'd gotten from mcdonald's. he's drinking from the cup. he strikes up a conversation, a casual conversation. >> reporter: the suspect and the undercover cop. >> the suspect and the undercover cop. obviously mark has no clue what's going on. as he's speaking with him, mark's car gets done and he gets up and leaves and leaves behind the coffee cup. >> reporter: police had their sample. and in less than 24 hours, they had the results. >> it's confirmed. it's a match. the same dna from the coffee cup is the dna from the latex glove. >> reporter: does your dashboard just light up when you hear that? >> can't even describe the emotion of it that we're going to be able to hopefully bring some resolution to the tyrrell family. >> reporter: two weeks later armed with search warrants, detectives arrested mark porter at his office. how do you take that? you got the wrong guy? this can't be? >> absolutely not. he said okay and turned around and put his hands behind his back. that was it, the only words he said. >> reporter: jessica was at work when she got the call. >> two of my very good friends were with me. he said he's been arrested. and i remember falling to the floor, and just being so overwhelmed with relief that he was finally caught. >> reporter: even the former secretary who didn't like him was horrified. >> i couldn't believe it. i mean, oh. i thought, no. >> reporter: investigators went back to mark's wife and told her that her husband's own story put him at the scene of the crime. >> you really need to help us out here. >> but when does mark say he was there? >> the time they were killed. >> no. >> yes, ma'am. yes, ma'am. >> okay. still circumstantial. >> reporter: and that's exactly what mark porter's defense attorney seemed ready to argue. mark hired one of the best to plead his case. >> he has a spotless record. five college degrees and has an outstanding past of good reputation. >> reporter: at a bond hearing, the defense revealed its strategy. they would argue that there was nothing suspicious about mark's dna inside his best friend's house. he was a regular visitor to the house, porter. he would get with gary and they'd put on their gloves and handle the old coins and the objects and that's why the glove is there. jurors, it's benign. do you think that works as an argument in the real world of courtroom drama? >> potentially yes. >> reporter: but what about that suspicious coin sale? well, there was no way to know for sure the coins had come from gary's house. and as for the assumption that mark porter was a compulsive gambler hopelessly in debt. you had to wonder if he got behind with a bookie or the mob. did he owe money to some heavy people? >> that was a theory that we frequently talked about. there was never any evidence that we found that led to that. >> reporter: so the case of potential death penalty was not airtight. and when mark hired new attorneys who asked to talk about a plea deal, the prosecution was willing to listen. >> what are we willing to risk? to try to get the death penalty. can we achieve what we need to achieve without the risk of going to trial? >> reporter: could have walked. >> there's a chance of a hung jury and you have to do it all again. there's a chance they come back on a lesser crime. >> reporter: both sides agreed. mark porter pleaded guilty to two counts of second degree murder and was sentenced to life in prison. no eligibility for parole until he's 82. >> i'm still very shocked, very sad. i have a lot of relief that mr. porter's finally going to be behind bars for presumably the rest of his life. >> he has to live every day with what he did. every day he has to think about what he did. and all of the lives he destroyed. >> reporter: you're saying he has to live his life in a kind of remorse, but you haven't seen any sign that there's that remorse. >> that's the only bad part. but he still has to sit there. >> reporter: today jessica is doing her best to move on. in february of 2015, she and jason finally got married. >> we got married on valentine's day. while it was a great day, it was also very sad. my parents weren't there. >> reporter: larry took his brother's place and walked her down the aisle. >> she asked me to give her away, and what should have been my brother's job. >> reporter: kind of bittersweet moment. >> mm-hmm, yeah, that's true. it was an honor to do it, but it didn't feel right. >> reporter: to keep her parents' memory alive, jessica has set up a scholarship in their names. as for that offbeat museum of a home, it's still there with a loving daughter as caretaker. >> i still feel them there. they loved that house. my mom built that house. i grew up there. and so i can't let that go. >> that's all for this edition of "dateline." i'm lester holt. thanks for joining us. . male announcer: it's time for the hottest party in hollywood. - whoo, that was good. announcer: jane lynch hosts six of her famous friends... - you look good, though, and that's all that matters. announcer: for a night of games... - we're gonna win! announcer: good times... - that's the best i can give you. 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