But detectives . Reporter they decided the killer wasnt out there, but inside the house now, with one of their own under arrest the family was furious. You see these people . This is love. This is love and belief in this human being. Please listen to us. Reporter did investigators rush to judgment . How do you have any idea, beyond your own fevered imagination, that thats what happened . you dont have a clue i think that i do have a clue reporter was the real killer still out there . Im lester holt, and this is dateline. Tonight, keith morrison, with a killing in cottonwood. 3 f2 en esta muerte. Here in northern california. His senior year was almost done. This was the place he loved. The game he loved. People sometimes talk about having premonitions of things. Did you have anything like that at all . It was just another day. Just another one of my countless baseball games. Reporter it wasnt of course. Though no one knew it, as they whiled their way through a golden afternoon redding, california with its famous sundial bridge, its parks, its Middle America feel, is a whole different place, a different life, than the california whose reputation blares technicolor from the botoxed and tmzd narcissism 600 miles to the south. Drive a few minutes from redding, and youre on the ball diamond in a little place called cottonwood. Here, in the shadow of mighty mount shasta, an anchor. A constant. Just like the people casey knew would be in the stands. So, were they there . Yep, both there. Always there. Always there. Never missed a game. Reporter his parents, mark and karen duenas, side by side by the dugout as always. You know, they were together since they were what, 17 years old. They practically lived in each others skin. Exactly. Reporter his mother had brought the chocolate chip cookies she passed out to the players after every game and for which she was justly famous. His father had made sure he finished work in time for the opening pitch. What kind of parents were they . They were the best parents that you could ask for. Reporter this is jason, their eldest son. And here the photo of jasons graduation, with his proud parents, the day he became a fireman. They would do anything for us, and they did. Reporter mark and karen were from big mormon families. High School Sweethearts who married as teenagers and watched their own brood quickly expand all boys. Jason, jacob, tyler, troy and casey. Boys bursting with testosterone. It was a rowdy house. We had Football Games inside the house. We were egged on by our parents and they loved it. Reporter karen was a full time mom and, once the boys were old enough, taught part time at nearby shasta college. For decades, mark got up at 2 am to drive for ups just because it freed him to coach his sons teams come afternoon. And to spare his wife middle of the night disruption, mark slept in a separate bedroom. Had for years. My dad treated her better than, you know, anyone could treat their wife. My mom was the queen. They lived simply. They lived paycheck to paycheck. They never had a lot extra. Reporter son tyler. For them it was all about family . Yeah, always. Reporter and they all stayed as close as any family could be. When jason got married, he moved into the house right next door. Helped celebrate mark and karens Silver Wedding anniversary, as they all did. By 2012, they were grandparents many times over, had been married nearly 33 years. One thing i always looked up to and loved about their relationship was that they still dated. Reporter tylers wife tina. After that long sometimes you can lose that, and i i never saw them lose that. Reporter but change does come for everyone. And by 2012, in their early 50s, mark and karen were making some changes. Karen went back to school, nursing school. This was a plan they had talked about . It was the perfect situation. She got to stay home with the kids all growing up and now she was ready to start her career. If you think about it, a pretty dang good plan. He worked his whole life, you know . He could start his own business while collecting a pension. And then she would, you know, start her career in nursing. Reporter not so easy of course. Nursing school is a tough thing. Not just for karen, either. Her brother joe and his wife jackie. Well, her commitments changed. Having grandma here when you need or having mom there when you need her, she was going to have to study she was struggling with the classes. She felt like she should be doing better. But i remember a text that said, you know i just got back from the er and got a chance to work with people finally and i love it. And so she was on her way. She was really on her way. Reporter and so, that may the 4th, casey joined his parents at home after his ball games. Then, it being friday night, he and a friend went to see a movie the avengers. I think started at 9 15. And told my parents everything was fine. Do you remember the last thing to said to your mom . She handed me a 20 bill. And i thanked her and hugged her goodbye. Reporter it was late when casey returned. And almost immediately fell asleep. Then, what do you remember next . It literally felt like i put my head on the pillow and two seconds later doors open, dads at the door freaking out reporter and seconds later 911, whats your emergency . 3 f2 familia duenas estaba a punto when we return, a dad devastated. Is one tragedy about to become two. I remember him coming in and collapsing. Falling to the floor. Over weight for a really long time. And now im not. Im a Weight Watchers coach, all of us have lost weight with Weight Watchers and are now helping others do the same. You can log into your computer or your phone anytime and you can chat with me. We understand, weve been there. Were still going through it. You are not alone and you can do it. 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Woah go right, go left, go left stop now go. shouting lets go i gotta go can i go . Yup you can go. beeping alert woah there you go way to go lets go buddy, lets go anncr the ford fusion. We go further, so you can. [rob] so weve had a tempurpedic for awhile, but now that we have the adjustable base, its even better. [alex] when i put my feet up on this bed my stress just goes away. [evie] i go up. Heeeeyyy. [donna]our tempurpedic is the best thing in our house, cept for my husband. [lauren] wait,wait,where are you going . [announcer] visit your local retailer and discover how tempurpedic can move you. Reporter it was 12 30 am. Now, may the 5th, 2012. Casey duenas, home from a movie and late dinner with friends, opened the front door, unlocked as usual and moved quietly down the little hallway toward his bedroom. Had it not been so late, he might have stopped by his mothers room to say goodnight. But the house was dark. Everyone asleep. Or so it looked to an exhausted casey, who fell into bed and almost immediately into a deep sleep. And then, 25 minutes later dads at the door freaking out. Go next door go next door go get jason somethings happened to your mom. Go get jason go get jason go get jason reporter casey did as he was told. Running next door to his oldest brother jasons house. 911, whats your emergency . Reporter while his father mark, beside himself, called for an ambulance. Tell me exactly what happened . Shes i just came in the room. I heard her. And shes got blood everywhere everywhere. I dont know. I have no idea. You have no idea what happened . No. You dont know where shes bleeding from . No, i know. I mean, theres blood everywhere. Are you with her now . Yes. Oh, its in her chest theres a gash in her chest reporter and jason woke up to the horror two ways at once. Fire department alarm. Brother alarm. I was asleep. As a volunteer firefighter, i have a pager. The pager went off. And i could hear casey coming up the stairs yelling at me in a panic. Theres something wrong with mom. You got to come next door right now. Did you have any sense of what it was . I had no idea what i was going to walk into. Reporter what he walked into was a nightmare beyond dreaming. There was his father in full blown panic. And lying on the floor in a puddle of blood, was his mother. And the rookie firefighter, for an instant to his eternal regret, froze. If i wouldve come on that situation now, i wouldve immediately started cpr. There was a great deal of blood, so for somebody to live through that, that much blood, i didnt think it was going to happen. I believe i just missed her last couple of breaths. Reporter minutes later, the emts arrived and karen duenas was pronounced dead. The cause . Multiple stab wounds, including a massive gash to her chest. Not long after that, the young detective arrived. Logan stonehouse, a year under his belt as an investigator. But this would be his first big homicide case as lead detective. And lo and behold i didnt know what was i walking into until i saw a picture on the wall and realized whose house i was in. Yeah, you knew these people right . I did. I went to high school with the second oldest son, jacob. Reporter and here he found himself looking at the bloody body of jacobs mother karen. The lady everybody loved. The one who brought cookies to all the games at school. Who could have done this . Detective stonehouse had been on the force long enough to be well aware that Violent Crime was up around here. Some said significantly. Ever since californias overcrowded prisons nearby had to release potentially dangerous offenders, like drug addicts, desperate for money for a fix. So, was this a Home Invasion robbery gone wrong . The detective took a look around the house. He and fellow officers fanned out around the neighborhood. There were some footprints outside the property, right . There was. Theres a wood fence that surrounds that piece of the property. Theres a back gate back there. And there was a little trail that led out to a canal area. Reporter and the first patrolman on the scene told them he had seen a car speeding away as he raced toward the duenas house. Neighbors reported seeing two strangers nearby that evening. And here, in this crime scene video taken the next day, a screen on the window in karens bedroom that looked suspiciously like it had been cut open. Had intruders entered here . Was anything disturbed around the house . No, actually the bedroom was pretty much intact. There wasnt drawers taken open like someone had burglarized the place. Reporter also, even though the screen appeared to be cut, didnt look like anybody had actually gone in that way. And so the detective had some questions for the only man known to be in the house all evening the husband, of course. Mark duenas. What was his story of what happened that night . He told us that he had been up with karen. They were watching a movie and then once karen went to bed, he tried to stay up a little bit longer to watch a giants game. He was falling asleep during it, so he decided to go to sleep himself. Reporter mark in his room, karen already down the hall he said in hers. The detective took mark to the Major Crimes Unit where mark, whod been awake more than 24 hours by now, finished the story himself. Im in my room asleep. I hear crazy noise. And you, you know, you hear what cats sound like . Uhhuh. It wasnt cats. It was a weird scream. Hear any talking or no, i just heard some weird screaming type stuff. And then whatd you do from there . I ran. I got up and cause i didnt want those, whatever was going on to wake up my wife cause i you know, im sensitive like that. And i went to the door by the kitchen, opened it up and looked out there. I didnt see any cats. Shut it, walked down the hallway, looked under, the lights were on. So, i figured shes up. Opened the door and thats when i found her. Reporter the detectives questioned mark for more than 3 hours, had him change so they could keep his shorts and tshirt for testing, then sent him home to his children. I remember him coming in the door and just collapsing. Falling on the floor. We were really worried about him because he kept saying like, his heart. And so we thought maybe he was having a heart attack. I mean, he was a mess. Reporter as the duenas family planned a funeral, their big extended family, including karens brother joe and wife jackie, descended on cottonwood. Ive never seen a person more broken. What was your perception of how he took it . Hes taken it incredibly hard. He still does reporter and along with unbearable grief for the family, came anxiety. I live next door. Im worried for my family. Im putting alarms on every door because somebodys out there. Reporter but the police did not seem to think that. In fact, within a day or two, one of the detectives announced no need to worry about some unknown killer stalking the town. Now, why would he say that . 3 f2 y por que lo diga . Veremos what did his story mean . When dateline continues. On the back of your shoe there. A price tag danger price tag alert oh. Hey, guys. Price tag alert is this normal . Well, progressive is a price tag free zone. We let you tell us what you want to pay and we help you find options to fit your budget. Where are they taking him . I dont know. This seems excessive decontamination in progress. I dont want to tell you guys your job, but. Policies without the price tags. Now, thats progressive. It may seem strange, but people really can love their laxative. Especially when its miralax. It hydrates, eases, and softens, to unblock your system naturally. So you have peace of mind from start to finish. Love your laxative. Miralax. Lifes morning multitasking. Its multiple ideas for growing families and drawers with many layers to show exactly what you need. Lifes the food that brings us together. And kitchens where every meal is the most important of the day. Murdered. The best way i can explain my mom is, she was pure of heart is the best way to say it reporter the whole big family, marks relatives,as well as karens parents, brothers, sisters, gathered to support each other and mourn and struggle with the question that hung in the air. Who did this . But the word on the street was that there was no question at all. We immediately heard rumors, outlandish rumors, and youre just going i dont know where youre getting this. Reporter the boys heard them too. Rumors that detectives knew the killer was mark to which the family said its ridiculous i know theres no way possible he couldve done it. My parents were best friends, from as long as i can remember. Reporter but if that was true then why would detectives be so suspicious of a husband married to his High School Sweetheart for almost 33 years . A man without so much as a traffic ticket on his record, let alone a violent act . Well, its true that when wives are murdered, husbands are frequently implicated and mark did discover the body. But there was another reason. Just minutes after mark sat down to talk to those detectives he volunteered information that sounded to them like a motive for murder. This is, its, what was going on in our life. And its it scares me when i think about it. Reporter it was an odd little story. One day at work toward the end of 2010, said mark, a ups coworker asked him if he had ever gone on facebook. No, said mark, he hadnt. Well, asked the colleague, havent you ever wondered what happened to people you used to know . Facebook might tell you. And mark thought about it and said, well, there was this girl he used to talk to back in high school. Wasnt a girlfriend or anything, but he was curious about whatever happened to her. So, sure enough, the colleague found the woman on facebook. And pretty soon she and mark were texting and talking on the phone. Catching up. We talked and we visited you know we kinda got carried away with a little texting here and there. Reporter carried away . Well, that may be a little strong. In this age of sexting and lurid electronic dalliances they didtalk and text, yes. For several months. Even said i love you a time or two, but was it some kind of affair . Well, judge for yourself. Was there ever any pictures sent or anything like that or was it always just theres pictures of just maybe, me or her, yknow just little, innocent, yknow were not into any of that nasty porn, none of that. It was just, what do you look like now . yknow its she had sent a picture of her and she goes your turn. she does a quilt or she took a picture of her quilt, showed it to me innocent stuff. She had a picture of her and grandkids and stuff like that. Reporter and, not once did they try to see each other, in fact, when an opportunity came up, they decided, no, it wouldnt be fair to their respective spouses of more than 3 decades. Or their kids. Or grandkids. So they didnt. But, as mark told the detectives, karen found what hed been doing when she went through a phone bill. And it hurt her feelings. So he promised hed stop. Except. He didnt. Mark went and bought a secret phone to continue that communication that karen wasnt aware of. Reporter it was the woman who put a stop to it. And she sent karen this letter, postmarked just a little less than 3 months before the murder, asking for forgiveness, promising it would never happen again. Karen told a couple of her kids about it all oh, she was pretty upset for a while. Reporter how important was that . I would definitely say everything together showed that karen was not happy in their relationship. Reporter mark told them otherwise, that it was a happy marriage, the texting thing just a blip, but the detectives didnt believe it. So they confronted mark. When did you find out karen was gonna leave you mark . Leave me . Uhhuh she was never gonna leave me. Never gonna leave me. Honestly reporter in fact, investigators believed it all backed up what they suspected from that very first night. Here, just hours after the murder, when they first accused mark of killing her. I didnt do it. And i i mean its crazy okay. And its just like o. J. We need to be out there finding the real killers, right . Lapd they didnt have to go find the real killers. I know youre doing your job. But i would never lay a hand on my wife. I did not hurt her. I did not kill her. I walked in and found her in her condition. Reporter marks family, by the way, said he told them all about that texting re