but as shock set in, leads poured in. witnesses reported strangers lurking. a car speeding through the neighborhood. and there were footprints out behind the house. >> i'm putting alarms on every door because somebody is out there. >> but detectives, they decided the killer wasn't 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 belief. in this human being. please listen to us. >> did investigators rush to judgment. >> do you have any idea, beyond your own fevered imaginations that that what happened? you don't have a clue! >> i think i do have a clue. >> was the real killer still out there? >> hello, and welcome to dateline. an unexplained death can fray even the strongest family of ties. but when tragedy struck the buenos household, grief pulled them closer. clues at the crime scene seemed to tell one story. but as investigators dug deeper, they wondered if this loving family had a killer in their midst. here is keith morrison with a killer in cottonwood. >> on the afternoon of may 4th, 2012, casey duenas walked out onto the field for the start of his very favorite thing, high school baseball. the double header. spring was in it's full glory 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 premonition's of things. did you have anything like that at all? >> it was just another day, just another one of my countless baseball games. countless baseball games. >> it wasn't, of course. though no one knew it, as they wiled their way through a golden afternoon. reading, california, with its famous sundial bridge, it's parks, it's middle america feel. it's a whole different place, a different life, then the california whose reputations blairs technicolor from the botox and tmz narcissism with 600 miles to the south. drive a few minutes from redding, and you are on the ball diamond in a little place called cottonwood, here in the shadows of the mighty mountain, an anchor. just like the peaceful people casey knew would be in the stands. >> so, were they there. >> yes, both there. >> always there? >> always there. never missed a game. >> his parents. mark and karen duenas, side by side by the dugout, as always. >> they were together since they were, what? 17 years old. >> your parents basically lived in each other's skin. >> exactly. >> 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 where they? >> they were the best parents that you could ask for. >> this is jason their eldest son, and here is a photo of jason's graduation with his proud parents. the day he became a fireman. >> they would do anything for us and they did. >> mark and karen were from big mormon families, high school sweethearts. who married as teenagers. and watch their own brewed quickly as 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, they loved it. >> karen was a full-time mom and watched the boys grew old enough, top nearby at shasta college. for decades, mark got about 2 am to dry for ups. just because it freed him to coach his son's teams african afternoons. and mark slept in a separate bedroom for years. >> my dad treated her the better than anybody could treat their wife. my mom was the queen. >> they lived simply. they live paycheck to paycheck, they never had a lot of extra. >> for them it was all about family? >> yes, always. >> 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 karen 's silver wedding anniversary as they all did. by 2012, they were grandparents many times over. had been married in nearly 33 years. >> one thing i always looked up to and loved about their relationship was that they still dated. >> tyler's wife, tina. after that long >> sometimes you can lose that. and i never saw them lose that. >> the change does come for everyone. and by 2012, in the early fifties, mark and karen were making some changes. karen went back to school, nursing school. >> this was a plan that they 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 good plan. he worked his whole life. he could start his own business while collecting a pension. and then she would start her career in nursing. >> not so easy, of course. nursing school was a tough thing. and not just for karen either. her brother joe and wife jackie -- >> well her commitments changed. having grandmother when you need her, or having mom there when you need her, she was going to have to study. >> as she was struggling with classes. she felt like she should be doing better. i remember dad said, i just go back to it from the er, and they got the chance to work with people finally. and i love it. so, she was on her way. she was really on her way. >> so, that made the 4th, casey joined his parents at home after his ball game. then, it being friday night, he and a friend went to see a movie, the avengers. >> it started at 9:15 and i told my parents, everything was fine. >> do you remember the last thing you said to your mom? >> she gave me a 20-dollar bill. and i think i thanked her goodbye. >> it was late when casey returned. and almost immediately fell asleep. >> what do you remember next? >> it literally felt like i put my head on the tip pillow and two seconds later the door is open. dads out the door freaking out. >> and seconds later -- >> 9-1-1, what's your emergency? >> the duenas family was about to discover that perfectly ordinary evening had ended in tragedy. coming up. >> she's got blood everywhere. everywhere. >> a dad devastated. is one tragedy about to become two? >> i remember him coming in the door just collapsing. >> when dateline continues. kayaking is my thing. running is awesome. but her moderate to severe eczema would make her skin so uncomfortable. now i'm staying ahead of it. dupixent helps heal your skin from within. so, they can have clearer skin and less itch. serious allergic reactions can occur that can be severe. tell your doctor about new or worsening eye problems such as eye pain or vision changes including blurred vision, joint aches and pain, or a parasitic infection. don't change or stop asthma medicines without talking to your doctor. ask your doctor about dupixent. a mystery! jessie loves playing detective. but the real mystery was her irritated skin. so, we switched to tide pods free & gentle. it cleans better, and doesn't leave behind irritating residues. and it's gentle on her skin tide free & gentle is epa safer choice certified. it's got to be tide viruses and bacteria float all around us. unlike air fresheners, only new lysol air sanitizer kills 99.9% of viruses and bacteria in the air. because scent can't sanitize. lysol can. meet the portable blender we can barely keep in stock. blendjet 2 gives you ice-crushing, big blender power on-the-go. so you can blend up a mouthwatering smoothie, protein shake, or latte wherever you are! recharge quickly with any usb port. best of all, it even cleans itself! just blend water with a drop of soap. what are you waiting for? order yours now from blendjet.com before they sell out again! here's why you should switch fro to duckduckgo on all your devie duckduckgo comes with a built-n engine like google, but it's pi and doesn't spy on your searchs and duckduckgo lets you browse like chrome, but it blocks cooi and creepy ads that follow youa from google and other companie. and there's no catch. it's fre. we make money from ads, but they don't follow you aroud join the millions of people taking back their privacy by downloading duckduckgo on all your devices today. >> it was 12:30 am, now may 5th 2012, casey duenas came home from a movie and late dinner with friends, open the front door, and move quietly down to the hallway towards his bedroom. had not been so late he might have stopped in his mother's room to say goodnight, but the house was dark, everyone was asleep, so it looked, when exhausted casey who fell in bed and almost immediately into a deep sleep. and then, 25 minutes later. >> dads out the door freaking out, go next door, go get jason, something has happened to your mom, go get jason, go get jason. >> casey did as he was told, running next door to his brother's house. >> 9-1-1 your emergency? >> while his father beside himself called for an ambulance. >> tell me exactly what happened. >> she's -- i just came into the room, and she has blood everywhere. everywhere. i don't know -- i have no idea. >> you have no idea what happened? >> no. >> you don't know where she's bleeding from? >> no, there's blood everywhere. >> are you with her? now >> yes. it's in her chest. there's a gash in her chest. >> and jason woke up to the horror two ways in ones, fire department alarm, brother alarm. >> i was asleep, the pager went off and i could hear casey coming up the stairs, yelling at me in a panic. there is something wrong with mom. you have 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. >> 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 internal regret froze. >> if i would've come on in that situation now, i would immediately started cpr. that was a great deal of blood. so for somebody to live through that, that much blood, i didn't think that it was going to happen. i believed i had just missed her couple of breaths. >> minutes later, emts arrived, and karen duenas was pronounced dead. the cause? multiple stab wounds including a gash in her chest. not long after that, the young detective arrived. 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 did not know when i was walking into until i saw a picture on the wall and realized whose house i was in. >> you knew these people, right? >> i did. i went to high school with the second oldest son, jacob. >> and here he found himself looking at the bloodied bloody body of jacob's mother. the one who brought cookies to all the games at school. who could have done this? >> detective stone house had been on the force well enough to know that violent crown was up around here, ever since california's overcrowded prisons nearby were forced to reduce population, and had to release some potentially dangerous offenders like drug attics, possibly desperate for money for a fix. so was this a home of asian robbery gone wrong? the detective took a look around the house, he and fellow officers got around the neighborhood. there were some footprints outside the property right? >> there was, there was a wood fence that surrounds the property and that was a back gate and there was a little trail that led out to a canal area. >> and the first patrolman on the scene told him that he had seen a car speeding away as he raced towards the duenas house. neighbors reported seem to strangers nearby that evening. and here, in this crime scene video taking the next day, the screen on the window in karen's bedroom that looks 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 wasn't drawers taking open, like some of the burglarized places. >> also, even though the screen appear to be cut, it didn't 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 stayed up a little bit longer to watch a game and he was falling asleep during it so he decided to go to bed. mark, in his room, karen, already down the hall, he said, in hers. the detective took marked to the major crimes unit where mark, who had been awake more than 24 hours by now, finished the story himself. >> i'm in my room asleep, i hear crazy noise -- and you know what a cat sounds like? it wasn't a cat it was a weird -- >> did you hear talking? >> no i just hear some weird screaming sound. >> and what did you do? >> i got up, because i didn't want whatever was going on to wake up my wife, because i'm sensitive like that, and i went to the door by the kitchen, opened it up and looked out there, i didn't see any cat. i shut it, walk down the hallway, and the lights were on, and i figured she was up. opened the door, and that's when i found her. >> the detective question mark for more than three hours, had him change silky they could keep his clothes and then sent him home to his children. >> i remember him coming into the door and collapsing. >> we were really worried about him because he kept saying his heart -- like, we thought that he was having a heart attack. he was a mess. >> as the duenas plan the family, the extended family, including karen's brother joe and jackie, descended on cottonwood. >>i've never seen a person more broken. >> what was your perception of how he took it? >> he took it incredibly hard. and he still does. >> along with unbearable grief for the family came anxiety. >> i live next door, i'm worried for my family. i'm putting alarms on every door because somebody is out there. >> 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? coming up. >> investigators were about to hear a strange tale from none other than karen's husband. >> it's scary when i think about it. >> what did the story mean, when dateline continues. is in for a surprise. meet arexvy. 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[bell dings] for a great low rate, go with the general. >> they measure time in units of pain in little cottonwood, california. during the surreal minutes and hours and days after karen duenas was murdered. >> the best way i can explain my mom is, she was pure of heart, is the best way to say it. >> the whole big family, karen 's family as well as mark's. parents, brothers, sisters, gathered to support each other and mourn. and struggle with the question that hung in the air -- who did this? word on the street was there was no question at all. >> we immediately heard rumors, outlandish rumors. i don't know where you're getting this? >> the boys heard them. to rumors the detective knew who the killer was mark. to which the family said -- >> it's ridiculous. i know there's no way possible he could've done it. >> my parents were best friends from as long as i could remember. >> but if that was true, then why would detectives be so suspicious of a man 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, it's 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. >> what was going on in our life -- and it scares me when i think about it -- >> it was an odd little story. one day at work towards the end of 2010, said mark, a ups coworker asked him if he'd ever gone on facebook. no, said mark, he hadn't. well, asked the colleague, haven't 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. it was in a real girlfriend or anything, but he was curious about what ever 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, we visited, you know. then we got carried away with a little texting here and there. >> carried away? well, that maybe a little strong in this age of lured electronic dalliances. they did talk 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 -- >> there is pictures of this, maybe -- her, innocent -- you know, we're not yet into any of that nasty -- it was just, what do you look like now? and she would send a picture of her, and she would i would return? she would take a picture of her quilt, to show it to me. it was innocent stuff. she had a picture of her and her grandkids. stuff like that. >> and not once did they try to see each other. in fact, when an opportunity came up, they decided it wouldn't be fair to their respective spouses of more than three decades. or their kids or grandkids. so, they didn't. but as mark told the detectives, karen found out what he'd been doing when he went through if she went through a phone bill and it hurt her feelings. so he promised to stop. except he didn't. >> mark went and bought a secret phone to continue that communication. and karen was unaware of. >> it was a woman who put a stop to it and she sent here in this letter, postmarked just a little less than three months before the murder. asking for forgiveness, promising it would never happen again. karen told a couple of her kids about it. she was pretty upset for a while. how important was that? >> i definitely say everything together showed that karen was not happy in the relationship. >> mark told them otherwise, that it was a happy marriage. the texting thing just a blip. but the detectives didn't believe it. so they confronted mark. >> when did you find out that karen was going to leave you, mark? >> leave me? she was never going to leave me. never going to leave me. honestly. >> 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 accuse mark of killing her. >> i didn't do it. i mean -- it's crazy. >> just like o. j. -- we need to go out and find the real killers, right? lapd didn't have to go find the real killer. >> i know you're doing your job, but i would never lay a hand on my wife. i did not her, i did not kill her. i walked in and found her in her condition. >> mark's family, by the way, said he told them all about that texting relationship. they said, it didn't seem like such a big deal. >> it sounds to me that it was just them confiding in each other about their marriages and their families. you know, just an outlet. >> for youngest son casey, it explained why there had been some tension between his parents a few months before the murder. >> there was a good span where they didn't get along. but it was a while before. >> yes. and they seemed to be back together again? >> they seemed better than ever. >> and of those claims by investigators about a pending divorce? not a chance, said the boys. >> they never were going to give up. they taught us that. i'm sure a little bit of trust was lost. someone keeps something from you. but, it was something they worked on. and they got over it. >> all of that grief infected summer. a cloud darkened over the duenas family. when casey graduated from high school in june -- >> i remember walking to where we would be seated and just seeing, my dad and mom along with other family members. but there was one spot missing. >> five months went by that way. as if those early police suspicious had faded away. but of course, we know better. don't we? >> coming up. mark duenas is in for a bitter shock. >> he was upset. like, are you kidding me? >> and what his family did next would surprise everyone. >> so this hardly ever happens. >> yeah, like never! >> when dateline continues. s around you, i think. my wife smoked, and she quit. because she needs to be here for to take care of me. 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>> they were not. detectives had suspected mark all along and now they had a warrant for his arrest. mark duenas was charged with first degree murder, held on a 1 million dollar bail to await trial. >> to see him in that situation was surreal. and you hope the system works because there is an innocent man up they're having to go through this. >> the whole extended clan including karen's family told anybody who would listen that the police made a huge mistake. >> i had never called anybody a name, he is not that kind of guy. >> i can't think of one time when he raised his voice to anybody. >> the family support is what's persuaded an attorney to take the case. ron powell was his name. >> at first i'm thinking, i don't want to go up there, but when i saw her family saying, can you help us, that type of stuff, and they meant mark, that's when i started to think -- >> victims family wants the accused represented by you because they believe he's innocent? >> correct. >> that is a once in a while thing? >> it never happens. >> but 14 months after the murder when mark went on trial, the family, his karen's and their's filled the galleries every day to support him. >> we were there because we know that person. >> the judge allowed cameras in court but no audio, so we can't play you the testimony from that woman from idaho who decided to conceal her identity, that's the one that he texted but hadn't seen in more than three decades. still, the state said that mark killed his wife. >> at some point she said that mark had mentioned that if they were meant to be together something bad would have to happen. >> the woman didn't seem to know what mark meant by that, but prosecution claimed that he would have to kill karen, not just because he wanted to be with her but because he wouldn't have to share his pension with karen, his wife of 33 years. karen must of been angry at mark. and might have told him that night that she intended to leave and she wanted to divorce. >> the facts of the case show that something happened that night between mark and karen whether that be her discussing divorce or whatever the case may be and he became upset and murdered karen. the prosecution based that theory around a story told by some teenagers two or three blocks away, who said they heard a screen between 10 and 11 pm. the pathologists testified that karen could have been killed as early as 10:30. or as late as 2:30, am. the state said that it must have been 10:30. after which, mark must of washed his clothes, slashed the screen to make it look like an intruder came in, then went to bed. and waited for son, casey, to come home from the movies. >> one of the theories that we did have is that he wanted casey to find karen. >> and he is sleeping peacefully, is that the idea? >> correct. >> and to cover his tracks? >> correct. >> except of course, casey did not discover the body, so mark had to make the 9-1-1 call himself. and when detectives went back and listened to that call, they heard what sounded to them like an incriminating mistake. >> 911 your emergency? >> there's blood everywhere. >> what did he say? >> i killed my wife. the state claims. then the sound they say is a well-known barnyard expletive, an unintended confession caught on tape. or so said the prosecution. >> so he got it all together and all planned and he blew it on the 9-1-1 call? >> i believe so. i'm sure if you just killed your wife you would be pretty stressed on the inside which would make something come out that you didn't mean to come out. >> but to the defense, it was quite simply bunk. >> i've listened to it i don't know 100 times, i don't hear that. >> defensive ron -- told the jury, mark found his wife just after someone attacked her and told 9-1-1 this, 9-1-1 your urgency? >> i gotta, i killed my wife, there is blood everywhere. >> i found my wife. and then that sound the state claimed was an accident, was really the word sick. >> i found my wife sick. >> do you think if they heard it that clear that this guy said i killed my wife and he's the only one home, you don't think they're arresting this guy, if that had happened, what are they waiting for they no rest i'm tool for months later? >> the police failed to follow up on the evidence, and intruders could certainly could have been surprised by karen, children then fled. there was an unidentified car park nearby, another car scene speeding away from the neighborhood as the cops raced into the house. the two strangers seen in the neighborhood had a trail of footprints leading away from the duenas backyard and, as for the screams heard by teenagers a couple of blocks away -- >> the entire block where mark lives hears nothing, the woman next door, sleeps with her window open here is nothing, the person on the other side of the house is jason duenas, and he hears nothing. >> karen told her the very week of the murder that she and mark were making plans for a bright future together. and the idea that mark would kill karen so he could pursue a happily married woman from idaho he hadn't seen in more than 30 years was simply ridiculous, said the defence. >> it sounds like a great motive when you look at it from a distance but then when you get to it, it sounds like puppy love. >> the defense rested its case in a matter of hours. >> when it went to the jury, how did you feel about that? >> i was pretty confident. you could see in the jury's face that they weren't buying it. >> then something odd happened. the day the jury went out a female juror overslept, and rather than delay the case, the judge replaced her with an alternate. and yes, on such tiny wheels, fates turned. >> coming up. jurors get the case, and are not impressed. >> i thought the sheriff's to department did a terrible job. >> terrible job? >> terrible job. >> when dateline continues. ice works fast. ♪♪ heat makes it last. feel the power of contrast therapy. ♪♪ so you can rise from pain. icy hot. ♪ i have type 2 diabetes, but i manage it well. ♪ ♪ jardiance ♪ ♪ it's a little pill with a big story to tell. ♪ ♪ i take once-daily jardiance, ♪ ♪ at each day's staaart. ♪ ♪ as time went on it was easy to seee. ♪ ♪ i'm lowering my a1c. ♪ jardiance works 24/7 in your body to flush out some sugar! and for adults with type 2 diabetes and known heart disease, jardiance can lower the risk of cardiovascular death, too. jardiance may cause serious side effects including ketoacidosis that may be fatal, dehydration, that can lead to sudden worsening of kidney function, and genital yeast or urinary tract infections. a rare, life-threatening bacterial infection in the skin of the perineum could occur. stop taking jardiance and call your doctor right away if you have symptoms of this infection, ketoacidosis, or an allergic reaction, and don't take it if you're on dialysis. taking jardiance with a sulfonylurea or insulin may cause low blood sugar. ♪ jardiance is really swell, ♪ ♪ the little pill with a big story to tell. ♪ i'm jonathan lawson here to tell you about life insurance ♪ jardiance is really swell, ♪ through the colonial penn program. if you're age 50 to 85, and looking to buy life insurance on a fixed budget, remember the three ps. what are the three ps? 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>> terrible job in the investigation. >> in fact, the jurors we spoke to said that feeling was pervasive in the jury room. and the 9-1-1 call prosecutors claim was a confession -- ? >> the jury couldn't decide what he said. >> we listened to it, many, many, many times. >> 20 times at least. >> still couldn't tell? >> no. the jury took vote after vote, 10 to 2 for acquittal at one point. but one juror in particular was adamant about his guilty vote. which one? the alternate who replaced the woman who'd overslept. >> that juror said, he's guilty and he's going to have to prove that he's not guilty. and i'm not going to change my mind. i'm going to hang this jury. >> the judge declared a mistrial. what was that like? >> it was heartbreaking. we thought my dad would be home that day. >> afterwards, marking karen's extended family publicly pleaded, do not retry. mark let him go. >> you see these people, this is love. this is belief, in this human being. this is not question in our hearts. please listen to us and know that we love this man. >> so, was it over? oh no. not even close. and said shasta county d a assigned a new prosecutor to the case. stephanie brigitte. something to know about miss brigitte. up to this moment she had never lost a case. not one. her secret, preparation she says. she's very thorough. >> by the time you finish reading through it, in your preparation, what did you think? >> i didn't have any doubt that mark was the one that killed his wife. >> one big change in trial number two, the very first words out of the prosecutor mouth were the states version of that 9-1-1 call, i killed my wife. blood everywhere. no jury she declared could doubt the content of that call. >> and prosecutors offered yet another damning statement from mark himself during his first of three tape recorded interviews with detectives. which again, he willingly submitted to without the presence of an attorney. >> it's the only cut i saw. >> in his very first interview, claimed the state, mark slipped and made an admission about while describing the wound he said he found on karen's chest. >> like the guy knew what he was doing or something, because the way he cut her, that's the only cut i saw. and there was tons of blood. but there was none there coming out when i -- whoever did it. i don't know. >> what was that again? >> listen carefully. >> but there was none there when, coming out, when i -- whoever did it. i don't know. >> when i found her? or when i did it? no way to know what he might have said. but the state claimed it amounted to another quasi-confession. the second prosecutor also presented evidence the first prosecutor chose not to use. testimony for example, from a criminal list who examined the clothes mark wore that night and found karen's blood, though not visible to the naked eye, all over. >> there is a big area on the front of his shirt that covered all the way down. there was blood on the back of the shirt. there was blood all across the waist band, down the sides, different spots throughout the shorts. even blood on the inside of the boxer shorts. >> which you don't think you could've gotten he could've gone there by touching his hands, moving, changing, adjusting? >> absolutely not. not in all those locations. >> the prosecutor claimed mark must-have washed off some, but not all of the blood. after the stabbing. like a shower in his clothes or something. then she claims, she found the murder weapon. or what could be the murder weapon. a knife found in the wrong slot in the kitchen butcher block. mind you, there was no blood on it. but it's handle was bearing substance identified as either animal fat or some kind of cleaner. >> what it tells us is that he had something in the home, available to him, that could have caused that murder. >> so, means, motive, opportunity. for premeditated murder. but would a second set of jurors agree? 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(both) the grill! thank you! it's more than soap. it's powerwash. >> november, 2013. a chilly autumn wind played around the corners of the big courthouse in the middle of redding. and inside, defense attorney ron powell fretted that he'd been short of a certain advantage. because now the prosecution knew his case. and besides, it had these new wrinkles. the blood on marks clothes and the knife in the kitchen butcher block. >> remember this is a retrial now. they felt this time that they needed to show a possibility of the knife. they never said this was the knife. they're just showing this knife could've done it. >> in fact, said powell, the new evidence was no more persuasive than the old. no blood, that animal fat found on the knife, how about somebody use the knife to cut a steak and then washed? >> could you wipe all the dna off a knife you put -- >> good question, i've learned now after this trial that blood will never leave clothes. but it will go right off the knife. >> as for the blood on marks clothes, of course there was blood, said the defense. he handled her body. and police didn't see the blood at first because, his shirt was read. and his shorts were black. and then there was the new prosecutors insistence that karen was very unhappy in her marriage and wanted a divorce. >> was it true. ? >> no, there's nothing to support. there's no facts to support it. there's no evidence to support it. >> this was the issue that went right to the heart of the prosecutor's case. and her contention that the state had developed a cure clear idea of just what happened the night karen was murdered. >> i believe that most likely karen confronted him with information that she had about the affair he was having with the lady in idaho. she probably said that was it, she wasn't going to have it. she was going to get a divorce. that's when he decided, i'm going to get the knife, i'm going to -- >> do you have any idea, beyond your own fevered imagination that, that's what happened? you don't have a clue. >> well, you know, all the evidence points to that. so, yes i think i do. >> it doesn't point to that. we >> have a person that has been in a relationship with a lady in another state. >> let me stop you for a minute. first of all, you said a fear. now you said being in a relationship. he was doing what millions upon millions of americans have been doing since facebook came along. if they haven't seen each other for 30 years. you wouldn't really call that an affair? >> here's the big difference, it becomes an affair and crosses the line when you don't tell your spouse about it. >> you believe that he was obsessed with this woman? i >> believe he wanted a lot more out of that relationship than she did. was it >> was a pure fiction, what was it? >> what else could it be? i know she's got a law degree. i never saw a psychology degree. >> from mark and karen sons, the prosecutor psychoanalysis of their mother amounted to an insult. >> the da said things about my mom that were untrue. did they know my my mom better than i knew my mom? better than any of us? they act like they do. there's no evidence. >> what do you say to people whose reaction is, well, of course you are going to feel that way. this is his family. they've just lost their mother, they don't want to lose their father, too. >> life is so much harder protecting my dad. we would be moved on. we would know what happened. if we thought that was the truth. but we all know with our hearts that's not what happened. >> the jury in the first trial had been hopelessly deadlocked, came so close to acquitting mark. and sending him home to his family. the second jury, was back in less than 80. and their verdict? was wooden on family faces in the gallery's first row. >> i still hear the sounds of the boys right behind mark, crying. i still hear. that it was very tough. >> guilty, a first degree murder. >> sick. i felt like throwing up, to be honest. i was sick. >> i feel bad for the family members, because the family members, they're not going to be happy with the verdict. but at the end of the day it's karen who was killed. and karen who that verdict was for. >> what would you say to them? the big extended family out there that doesn't think they got justice at all? >> i would tell them that i am confident that the right person was convicted. >> mark duenas was sentenced to 25 years to life in prison. and his family? they'll go on believing what they have always believed. that late a night on may 4th, 2012, unknown intruders, probably drug addicts intent on theft, burst in, were surprised by karen, killed her and then realizing what they had done, ran away without taking anything. jason no longer lives right next door to the home he grew up in. in fact, the family recently and reluctantly sold the home. the place it happened. the memories, upkeep, too much to bear. >> i mean, it was always in cottonwood, small town. nothing happens here. and then the worst you can imagine happened. >> casey and his brother troye no longer play baseball for the local college. the days when they look towards the bleachers where their parents, always sat side by side, are gone forever. in fact, the family recently and reluctantly sold the home. the place it happened. the memories, upkeep, too much to bear. >> i mean, it was always in cottonwood, small town. nothing happens here. and then the worst you can imagine happened. >> casey and his brother troye no longer play baseball for the local college. the days when they look towards the bleachers where their parents, always sat side by side, are gone forever. >> that's all for this edition of dateline. i'm craig melvin. thank you for watching. >> and i'm natalie morales. >> and this is dateline. >> that night it's absolutely pitch dark. it was abandoned and it was secluded. this was his playground. this was the place we had been searching for. >> it was the still of the night when the killings started. >> i have been shot me and my mother have been shot. >> you can feel in here the fear in her voice. >> a single mother and her teenage daughter, ambushed in their bedrooms.