youtube. we just passed the 2 million mark. please join the growing community, and be a part of the conversation. i'm john quinones. "20/20" starts right now. what happened to my sister? how could somebody do this to my sister? >> imagine you're a mother. discovering your daughter under attack at home. >> he looked at me as he reached for the biggest butcher knife on the counter, and he said, you're next. >> it was someone that knew her, and passionately wanted to hurt her. >> one of my daughters said, do you think dad could have done this. >> we have an eyewitness. all the information was right there. >> then why couldn't they identify the mystery man for nearly a decade? >> finally a breakthrough, using

hypnosis. >> now, tape never seen before as we take you inside the investigation. >> she says, oh, my gosh, dad. his picture matches up. >> two mothers squaring off. one daughter a victim. one son, the main suspect. >> my son didn't do this. >> 0%? >> 0%. >> he's such a good man. he's so gentle. well, i'm sorry, he's not. >> so much hinging on a cutting-edge dna test that turns a drop of blood into a photo. >> there's no evidence putting him in that house. >> there was unknown dna evidence that excluded justin. >> tonight, which mother's instinct will be right? >> you don't believe you're guilty. >> i know i'm not guilty. >> good evening, i'm amy robach. david is on assignment tonight. reporting tonight, matt gutman. >> reporter: it's been said that our lives are built one memory

at a time. no experience ever really gone as long as we remember. but at the center of this story is a memory that was lost. here in albuquerque, new mexico, in 2008, life for 17-year-old brittani marcell was as carefree as the sky over this city's annual hot air balloon fiesta. she lived in this house with her single mother diane, one of seven siblings. >> tell me what life was like, 2008, in your house in albuquerque. >> i had at that time five kids living with me. two were on their own. and it was joyful. a beautiful family and a lovely life and i couldn't have asked for anything better. >> reporter: brittani fell right in the middle of her boisterous family. number five of seven. kathlein guinn and alicia marcell are two of her older sisters. >> she had a great personality. she would talk to anyone, very outgoing. everyone knew her for her positivity.

>> i know it's a cliché to say but she was going places. she had dreams and goals and she was taking care of them. >> reporter: one of those dreams, to be a television reporter. but for now the high school senior held down a more modest job at the local mall. >> she worked at a sunglass icon. talking people into buying them. >> reporter: the mall was just down the road from where the marcell family lived on the west side of albuquerque, a cozy swath of suburbs just off the rio grande. >> our neighborhood was perfect. it was nice. nothing happened like that. >> reporter: which made what did happen here on september 11th, 2008, so unfathomable. it was a thursday and brittani would be out of school early. >> she said, do you want to meet for lunch. i said, sure, where do you want to go? and she said, let's just meet at

home. okay, fine. >> reporter: and you called her as you were driving home? >> i did. >> reporter: and what did you say to each other? >> reporter: minutes later diane pulls into her driveway and walks into a terrifying scene. >> i unlocked the door and i see this guy with a shovel and she's over here to the left at the bottom of the stairway, bleeding and i thought really she was dead. >> reporter: diane can't believe her eyes. her daughter lying on the floor beaten and bloodied. her purse, shoes, and dvds are scattered just inside the door, dropped as she was hit in the head by a man wielding a shovel. but the assailant was right there, standing over her. >> and he looked at me and he just -- >> reporter: you made eye contact. >> right. he turned around and he, he had the shovel in his hand still. this guy takes off for the kitchen and he looked at me as he reached for the biggest butcher knife out of the knife block on the counter. and he said, you're next. >> reporter: diane runs frantically from the house screaming. a neighbor on the street comes to her aid as she dials 911. >> what is your name?

>> diane marcell. >> i said, my daughter's been beaten and the guy's probably still in there. >> i'm afraid he might be in there! oh, my gosh. this is a life or death situation, can they hurry? >> reporter: even before police arrive and unaware the attacker has escaped, diane is running back into the house with that neighbor, desperate to save brittani's life. >> can you come with me, please. i'm so afraid. >> reporter: the entire course of your life changed in seven minutes. >> it did. >> is your daughter breathing right now? >> she' breathing, but moaning, she's going to lose consciousness, please, there's blood everywhere. >> her face was already swelling. her head was swollen already from the beating. her arms were bruised. and she was moaning. >> we had a homicide call out, there was an individual on the west side that was attacked inside of their house. >> reporter: jason morales was a detective on the case. he's since retired from the albuquerque police department. >> myself and the other homicide

detectives responded to the scene. >> reporter: brittani is whisked away in an ambulance. detectives want to speak to diane immediately. what did they ask you specifically? >> they asked what he looked like and i told them. he's either a dark caucasian or a light hispanic i said he was a light hispanic, i said he was probably 5'7" or you know 5'11", in that range. probably in his 20s. >> reporter: investigators assess the scene, trying to determine what actually happened. >> the entire house had not been touched. >> reporter: detective morales collects the weapons left behind by the attacker. >> we had the shovel that he was holding when diane walked in, there was also duct tape at the scene, and then the fact that he had armed himself with a knife. >> reporter: and there was something else that attacker

left behind. >> the suspect actually went through a large living room window instead of exiting out of the sliding glass door. he cut himself, leaving his blood. >> one perfectly round drop of blood. a calling card inadvertently left behind by whoever did this. they had his dna. now all police needed to do was find the guy. as diane rushes to the hospital, the rest of the family gets word. >> when we saw her we realized how very, very bad it was. >> there was severe brain trauma. they said that she did try to fight him off because she had severe bruising on each arm at the upper part. her left wrist was broken, her jaw was broken. >> when we finally went in, we just dropped. "this is brittani, no." her head was like a huge balloon, face swollen, bruised, battered.

>> i had that terrible feeling because when the detective came he said he was a homicide detective and i was like, "no one thinks she going to make it." >> reporter: that detective needs diane's help again. >> now we need to go down to the station, make a sketch. the quicker we get it out, the quicker we can publicize it. >> this is the man police want to talk to in connection with a vicious beating. >> reporter: as police start fielding tips, brittani's family caonly pray she will somehow pull through. coming up -- who could have done this? >> we all thought it was someone that she knew. >> could that someone be closer to home than anyone suspected. >> one of my daughters said, "do you think dad could have done this?" >> reporter: stay with us. m rel. mbc doesn't take a day off, and neither will i. and i treat my mbc with new everyday verzenio- the only one of its kind that can be taken every day. in fact, verzenio is a cdk4 & 6 inhibitor for postmenopausal women

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>> reporter: on a clear september day in 2008 here in albuquerque, new mexico, 17-year-old brittani marcell was walking in her door when she was brutally beaten on the head by a man with a shovel. as detectives at the scene are piecing together the evidence, doctors are trying desperately to save brittani's life. they place her in a medically-induced coma for two weeks. but she does wake up. this is brittani marcell today. she's now 27 years old and says when she came to at the hospital, she had no memory of that terrifying day. was it scary not being able to remember anything? >> it was scary after they told me what had happened. >> reporter: so you at first didn't know why you were in the hospital? >> right. i thought i was in a bad car accident. they're like, "that's far from what happened to you. you were beaten traumatically, with a shovel, in your house."

it was hard to take in. very. >> they had to go in and remove part of the lower left temporal lobe of her brain. it's about a nickel size. they removed probably three by four, four by three part of her skull. >> reporter: wow. >> implanted that in her abdomen. >> reporter: that's a pretty big size. >> right. >> reporter: that's a big chunk of skull. >> they cut her abdomen and put it inside her abdomen. it healed inside of her abdomen. so she had to wear this helmet. >> reporter: eventually doctors would put the piece of healed skull back in brittani's head. they cover the mirrors in her hospital room, afraid of how she'll react to her own reflection. her close-knit family spends every waking moment at her bedside. >> we tried to get her out of bed and seeing her in the wheelchair and she was drooling all over herself, i was like, "what happened to my sister? i'm pushing her in a wheelchair and she physically cannot keep her head up. how could somebody do this to my sister?" >> reporter: what was it like to

see your daughter have to learn the things that you've already -- you'd already taught her when she was a toddler? >> it was very difficult. >> reporter: eating, walking. >> you'd cheer her on, you know, "you can do it. >> reporter: what were the doctors telling you? >> that she was going to have memory loss and it could be long term, it could be short term. just don't get your hopes up too high. >> reporter: but hope is all the family has to hold on to, that brittani can recover physically and maybe recover memories that could lead to a suspect. meanwhile, detectives still have that single drop of blood. they run it through the national dna database hoping for a match, but no luck. as the months pass, friends and family put up billboards and turn to the media for help. >> the person that did this to brittani still hasn't been caught. we wanted to let people know that he's still out there. >> the tips kept coming in. i'd call the detective probably every day, and the reward fund was going up and nobody.

>> reporter: the marcell home, once filled with so many happy memories, now too painful for diane to go back to. you never lived in that house again? >> i never lived there again. i couldn't do it. there was no question. >> reporter: instead she splits her time between nursing her badly injured daughter back to life and being her own investigator on the case. >> reporter: you were suspicious of basically everybody. >> i was. everybody was on my radar, i don't care if it was a friend, a neighbor, a co-worker, i want to know who did it and i wasn't gonna trust you. >> we all thought it was someone that she knew, just based on how passionate of a crime it was. >> reporter: but figuring out who would want to hurt the high school senior with the beautiful smile wasn't easy. she had no enemies. >> at points, i thought it was mistaken identity. >> reporter: diane begins to think perhaps she was actually the intended target. after all, she did go through a contentious divorce just six years earlier.

>> my ex-husband, he was a bitter person after the divorce and one of my daughters said, "do you think dad could have done this?" >> reporter: he wasn't just bitter, he was violent during your marriage. >> i got out of a marriage because of domestic violence. >> reporter: did you know that he had been violent towards her? >> i did. as a kid i -- i distinctly remember that. and it was something you shouldn't remember. maybe he, you know, had hired somebody at one point because he didn't like me anymore and they got the wrong person. >> reporter: but her suspicion would soon turn into more disappointment. >> we were able to speak to him and kind of figure out where he was, anybody that he might have been dealing with, and that information never really panned out. >> reporter: brittani's dad is officially cleared. and for ten months, brittani continues to improve, relearning to walk, talk and swallow on her own, eventually even earning her high school diploma from a rehab facility where she lives after leaving the hospital. as you're seeing this amazing progress with your daughter in her rehabilitation, you're not seeing progress in the investigation. >> right. i just thought, "well, they're not going to find him."

>> you did start to worry that the case may not ever be solved. >> reporter: david waymire is a deputy district attorney in albuquerque. >> unless brittani marcell recovered memories of the attacker, that the one and only piece of evidence was a single blood drop. >> reporter: that's right. the whole case hinges one drop of blood. so two years after the attack, waymire makes a bold move. to get around the statute of limitations, he indicts the dna itself from that drop of blood, referring to it as john doe. >> he said, "whenever this person's arrested, his name will attach to it. right now, is going to be a john doe, because, you know, we know it's a man." >> reporter: but a name is what investigators needed to close this case, and diane marcell thinks a new detective could breathe new life into this case. >> talked to the sergeant, i said, "when can we get a new set

of eyes on it?" she said, "i got somebody for you." she told me, "i'm going to find the person who did this." >> reporter: and her out of the box approach is about to turn this cold case red hot. >> she said, "well, let's just try something new." >> reporter: and it turns out hypnosis would be the key to unlocking brittani's memory. >> in the video, and she's remembering what happened. she's giving details. >> reporter: next. my speechless. so here is charles barkley to speak for him. how'd they make a burger this terrific four words: cooked. when. you. order. that's right. they don't cook it until little jimmy tells 'em to. little jimmy doesn't see cheese that melty on a burger every day. and boy is it juicy. shhhh.... little jimmy, don't talk with your mouth full, it sounds terrible. the new hotter, juicier fresh beef quarter pounder burgers. so good, they'll leave you speechless. ♪ happiness is powerful flea and tick protection from nexgard. a delicious chew that protects for an entire month.

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>> reporter: with her daughter's memory of the attack still a black hole and the original detective retiring, she asks for a fresh set of eyes. >> i was always interested in criminal justice. and then in 1999, my sister was murdered. >> lisa kendrick was gunned down on mother's day, 1999. >> so that pretty much set off my path to being a homicide detective. >> reporter: detective jodi gonterman has been with the albuquerque police department for 17 years and has a reputation for being relentless. >> the first moment i met jodie, she told me, "i'm going to find the person who did this."

>> i kind of have a little ocd. if i have something that needs to be done or finished, i have to complete it. >> reporter: and right off the bat, she has an unusual suggestion. something her department had never tried before. >> i wasn't so convinced that her loss of memory was due to brain damage. i thought it could have been that she was suppressing the traumatic memories that she had. she said, let's try something new. >> reporter: hypnosis. meet forensic psychologist dr. leon morris. he hypnotizes some patients to unlock repressed traumatic memories. that was literally the first time you met her. >> absolutely. all of my contact with her is on the video. tell me why you're here today. >> let's see. there was an attack about almost six years ago. brutally beaten on the left side of my head. do i remember any of that? simply no.

>> reporter: you didn't know in the beginning whether or not her memories were recoverable? >> i knew she had quite a bit of brain damage and i assumed that, you know, a lot of it was gone. but i -- in the interview, she talked about going to college. what was your grade point average? >> 3.7, 3.5, 3.7. i can't remember. >> so you did extremely well in college. and i thought, "well, wait a minute. she must have remembered something from her previous education. there's some memory there of some kind." >> reporter: after his initial interview, dr. morris puts brittani under hypnosis. this is moment when brittani begins shaking as she relives the attack. >> tell me what's -- what is happening. >> he's -- he's hurting me. >> how? >> with a stick. or something big. i'm bleeding. >> did he say anything?

>> mm-mm. >> not a word? >> you can feel it almost. like you're kind of reliving that. >> reporter: and you said, "stop, that hurts. please." >> i was like defending myself almost. >> when i saw it, it was heartbreaking, to know that she relived it in her mind. >> after that was done the doctor told me that she's going to probably start remembering now. >> reporter: and that's exactly what happened. detective gonterman follows up with brittani over the phone and lo and behold, she remembers even more detail about her attacker. >> he was a tall guy. from what i remember. i did not see any tattoos on him. >> do you remember if his shirt was long or short sleeves? >> it was short sleeved. >> i remember those eyes, though. they had to have been brown. >> reporter: the first fresh lead in years spurs detective gonterman to try something else new.

seeking out a cutting-edge dna test that promises to extract more information than ever from that drop of blood. >> they would take a dna profile, and they would give us hair color, eye color, ancestry. and then they do a 3d computer-generated image of what your suspect's going to look like. >> i was skeptical but it was an encouraging sign to attempt it, to do anything that could build on what we knew was our single piece of dna evidence. >> reporter: while detective gonterman waits for that computer generated image to come back, she gets a call from brittani. a name has popped into her head. one she says she can't stop thinking about. >> she said, "jodi, i remember the name justin hansen." she said, "i remember working at the kiosk at the cottonwood mall, and he would come by and visit me." >> reporter: but that's it. no memories specifically linking justin hansen to the attack.

he is seven years older than brittani and she says she met him years earlier through a friend. >> when i was in middle school, i had a best friend named abby, and she had an older sister named lauren, and lauren ended up getting pregnant by him, whenever i went to abby's house or so, he would be over with lauren. >> reporter: by the time brittani got to high school, she says her friendship with abby had soured. but every once in a while, when she was working at the mall, she says she would still see hansen. do you remember any of the conversations that you had with justin? >> they were just like, "hey, how's your day? how was school? what are you doing later?" a regular talk you'd have with anybody. >> i actually didn't think anything of it. >> there were so many names over the years, you just stop being like, "could this be the one?" >> reporter: but then that dna generated picture comes back and it's a bombshell. >> i thought, "wow, it looks so much like him."

>> she called with the news. and she said, "oh, my gosh, diane. you're not going to believe this." his picture matches up. >> and they indicated a high likelihood that the suspect would have either green or hazel eyes, a fairly uque eye color. >> repter: a uqueye color that just happens to match justin hansen's. >> so at that point justin hansen did jump to the top of the list. >> reporter: coming up -- detectives have a suspect, but there's a problem. he seems more like a proud papa than a predator. >> he didn't have a criminal history. >> reporter: and no apparent motive, but they track him down anyway. >> are you justin? >> i am justin. >> reporter: and play a clever game of cat and mouse. >> when is the last time you saw brittani? >> reporter: caught on tape. next. w light bladder leaks can impact life. so does poise®. from pads and liners that keep you comfortably protected, to impressa®, which helps stop leaks pad-free.

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>> reporter: last year the man who lived in this house, justin hansen, found himself in the crosshairs of one determined detective. that, after brittani marcell remembered his name and a new kind of dna test from that drop of blood generated a picture that looked an awful lot like him. >> detective brown and i, we went to the house and justin came outside, and he was in his bathrobe. >> i'm investigating a case, it's an older case. just going back and talking to people who knew brittani marcell. >> okay. >> reporter: the video you're watching is from a police body camera. justin hansen is at home with

his wife and three small children when detective gonterman shows up unannounced. >> he was very friendly, he answered my questions. what did you hear that happened to brittani? >> i heard that somebody attacked her that she could have been raped or something. i don't really know much, though. >> at the end of the interview, when i asked him, "can you give your dna." he changed. >> can i think about this and then come back and see you? >> if you know 100% that you were never even in that house there is no way it's going to match anything. >> could i get your card though and come back and you know just so i think about everything, talk to my mom. >> up to that point from 2008 to 2017, between 15 and 20 people had actually been interviewed as possible suspects, and all of those inviduals had volunteered to give dna.

>> here's my number. >> mr. hansen was the first individual that refused or declined to give dna. >> reporter: but before throwing in the towel, detective gonterman steps outside with hansen and ratchets up the pressure. >> we have a 3d composite and it does resemble you. and i would rather do this now before i put this out on the news media. >> like i said, just give me a day or so to talk to my mom and everything else. i got your number. >> reporter: but after a week with no word, detective gonterman tracks down hansen's mom, doreen shoemaker. we decided to do the same. >> if you follow the rio grande out of albuequrque you'll come to a place called bosque farms. this is where justin hansen grew up. >> my son didn't do this. >> reporter: 0%? >> 0%.

>> reporter: how would you describe him? >> a talker. justin is very friendly. he can start a conversation with a rattlesnake. >> reporter: doreen says her son couldn't understand why detectives seemed so fixated on him. >> he asked me, "should i be worried?" and i told him, "i don't know. were you involved at all?" and he's like, "no." and i said, "then you don't have to be worried." i know my son. he's never had aiolent history. >> reporter: even detective gonterman concedes that at first glance justin hansen doesn't fit the profile of a violent criminal. >> he didn't have a criminal history but when i started looking into him, people would tell me stories about him that were, to me, a red flag. there was a report that i came across where he was a suspect in a rape. it was a boyfriend, and he had entered her home, raped her. she decided that she didn't want to prosecute because she was

drinking at the time. >> they had been dating for a couple years. and she admitted that she had been drinking and -- >> reporter: none of that actually discounts an allegation of rape, though. you can be dating someone. and you can be raped. >> right. right. i understand that. she's the one that withdrew it. and she continued to date him. >> reporter: hansen has also denied the allegation of rape. and there were other women from hansen's past who tell detective gonterman they've never seen him be violent. >> there's a lot of cases that i've seen where you have the nice guy next door, he's the nicest guy, i never would have thought this. i think he fits into that category. >> reporter: but if that's the case, doreen wonders if brittani's mom diane also saw the attacker, why couldn't she identify him in a photo line-up? >> she said she'd never forget that face. and i know as a mom i would never forget that face. >> reporter: were you able to recognize justin? >> no. he was my second choice.

>> reporter: one would think -- >> mm-hmm. >> reporter: that if you had stared down your daughter's assailant, in your own home and you actually heard him speak, you saw him go to the kitchen -- >> right. >> reporter: it would it be, seared into your brain like you said. >> he looked, he looked and he went that way and my eyes went down on his hands to find out what he was doing and then i flew out of there. >> so you're saying that you didn't look at his face all that much? >> right. >> reporter: which could explain why justin hansen doesn't exactly match the original sketch. but what about that posthypnosis description from brittani? >> i remember those eyes, though. >> she was really clear that, you know, she saw the person. they had brown eyes. >> reporter: doreen points out her son's eyes are undeniably green. >> she describes what he's wearing.

does not mention tattoos. but he has a short sleeve t-shirt on. >> reporter: did he have tattoos in 2008? >> yes. >> reporter: detective gonterman remains undeterred by these inconsistencies. she's convinced that justin's dna will prove he is the person who attacked brittani if only she could get it. >> we had to find it. i met with some of our undercover detectives, and i asked them, "can you follow this guy and get his dna?" >> he was at a fast food restaurant drinking from a cup, and he threw that cup into the trash can. those undercover officers then obtained that trash, including that cup, took it to the crime lab where it was analyzed. >> reporter: after one agonizing month, detective gonterman walks into a meeting with the crime lab analyst. >> she handed me a folder, and i opened it up, and it was the

parabon snapshot report, it was justin hansen's photo, and she put match on it. i jumped up, i started screaming. that turned into crying. i was so happy, i just couldn't believe it. >> reporter: when we come back with the dna match in hand, it looks case closed. but not so fast. >> there was unknown dna evidence that actually excluded justin hansen. we knew that that would be a problem. >> reporter: so would the charges against justin hansen ever stick? how did that feel? >> i was livid. >> reporter: stay with us. cactu. (man 1) i read that the saguaro can live to be two hundred years old. (woman) how old do you think that one is? (man 1) my guess would be, about... (man 2) i'd say about two hundred. (man 1) yeah... (burke) gives houseplant a whole new meaning. and we covered it. talk to farmers. we know a thing or two because we've seen a thing or two. ♪ we are farmers. bum-pa-dum, bum-bum-bum-bum ♪

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>> reporter: after nearly a decade of searching, detective jodi gonterman now believes she can prove who that drop of blood

from the scene of brittani marcell's attack belongs to. before arresting justin hansen she calls diane marcell hoping to give brittani the news. >> she was still trying to get a hold of brittani the day of the arrest. she said, "where's brittani?" i said, "she's traveling." and she says, "do you want to tell her?" i said, "if you don't mind." >> reporter: brittani just so happens to be on a girls trip to boston. >> we get to the hotel and i set up my little iphone camera and she still has no idea what's gonna happen. and we call my mom. >> jody called me today and we have a match. >> really? >> yes, we do. off the cup from justin. >> no way! >> reporter: that same day officers tail hansen, watching him leave the gym, pick up his children from daycare and head to a grocery store. it's here that they swoop in to make the arrest. >> i don't know what the hell is

going on right now, my kids are probably terrified. >> reporter: detective gonterman is at the station to meet hansen. >> you feeling okay today? >> no. >> reporter: she tells him he's facing a laundry list of charges including attempted first degree murder. looking at the possibility of more than 50 years behind bars. and she has something else to say. >> look at her, justin. you did this to her. >> i didn't do that. >> you did this to her. >> i didn't do that. >> you can deny it all you want. i know it's you now. i'm not asking, i'm telling you it's you. >> brittani said, "well, they caught him, so he's just gonna go away forever." and i'm like, "that would be really awesome, but that is not the justice system." >> reporter: a week later the father of four is in a packed courtroom for a bond hearing. there are supporters on both sides. although they are no longer together, his wife alyssa does something you don't hear

every day. sing the praises of the man she's divorcing. >> justin's probably the most selfless person i've ever met in my life. >> she goes and testifies at the hearing that he's such a good man. you know, he's so gentle. well, i'm sorry, he's not. >> reporter: but with no criminal record prosecutors fail to convince a judge that hansen is enough of a danger to be held in jail without bond. >> mr. hansen, do you have anything to say to brittani and her family? >> did you do it? >> reporter: he was let go on bail. how did that feel? >> livid. i was livid. >> reporter: brittani says even though some of her early recollections of the attack may have been inaccurate, today she says she remembers the whole thing. >> i unlocked the screen door and i heard that big jolt on the back of my head, and i'm just like, "justin?" like, "why, justin, why?"

i remember that distinctly. >> reporter: but prosecutors know the accuracy of these memories will be challenged and that isn't their only problem. >> we were also struggling with identifying a motive. >> reporter: prosecutor waymire zeroes in on the fact that justin's number was found stored in brittani's phone. it's his best theory about a motive. >> so even though there weren't any recent calls, or texts, or anything like that the theory would've been that justin hansen was essentially stalking brittani even if nobody really knew it. >> reporter: but there's little actual evidence to support that theory. and when detective gonterman calls brittani's best friend from 2008 justine baca, she's never heard of hansen. >> i have no idea who that is. >> okay, and she never mentioned the name justin? >> mm-mm. our names are so similar, it always sticks in my head when i meet somebody that knows a justin. >> she's like, "i've never met this guy. if brittani was hanging out with him she wasn't telling me about it." >> reporter: justin's mom says the reason for that is because

her son never stalked brittani and isn't the person who attacked her. >> if somebody has that kind of violence in them, you know, you'd think if they can go a whole decade and it not come out again, i don't think so. >> justin was staying out of trouble. he knew that if he got arrested and his dna was going to get in the database, that he was going to get caught for this. >> reporter: so what about that dna evidence that seemingly puts hansen at the marcells' house the day brittani was attacked? what do you make of that one drop of blood? how did it get there? >> i don't know. i do know that they also had dna in the house on both weapons and it doesn't match justin. >> there was unknown dna evidence that, in addition to not being justin hansen, it actually excluded justin hansen.

>> they have zero evidence that he was in the house. >> reporter: so the only evidence they have is that one drop of blood. and you're saying that's outside of the house. they can't actually prove that he was in the house. >> correct. >> reporter: doreen shoemaker believes additional testing on those weapons could exonerate her son but that can never be done. back in 2015, when that original detective was preparing to retire, a clerical error led to the destruction of almost all the evidence in the case. >> that included everything from the shovel, the knife, the duct tape, a lot of the other dna evidence and blood swabs. >> it was an accident, i'm sure it was a mistake, but it happened. luckily the dna wasn't destroyed. >> reporter: one more challenge prosecutor david waymire will face before a jury. but doreen shoemaker says it isn't just those 12 people who should be concerned that the wrong guy could be taking the fall. >> i feel for the marcells. i couldn't imagine being in that

position. i know that brittani has suffered a lot. and i would think that they would want to be 100% sure that the person they're sending to jail is the person who beat their daughter. and there's no evidence putting him in that house. >> reporter: but prosecutor waymire says the evidence just outside the house, that drop of blood, is enough to convict. >> there is no explanation for that that can be reasonably offered, other than the fact that that was left at the time of the attack by the perpetrator himself as he broke that window and fled. >> reporter: you know, memory is really fluid. >> mm-hmm. >> reporter: are you certain that it's him? >> i'm 100% positive it's him. >> reporter: coming up, just as the case heads to trial, a twist that changes everything. >> i get a phone call from jodi. my first thought was, this guy's gone. >> reporter: you thought he bailed, he skipped town?

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albuquerque from texas for justin hansen's trial, she gets an unexpected phone call from detective jodi gonterman. >> she said, "he's gonna plea. they're gonna do a plea hearing today at 2:00." i know you can't be here, but call your daughters, get people there." >> the defendant agreed to plead no contest. >> reporter: hansen pleads no contest to attempted murder and aggravated burglary. prosecutor david waymire says this avoids the risk of a trial while ensuring hansen serves time. >> we knew that the jury would have questions on whether or not the single blood drop of justin hansen's in the absence of a concrete motive would really be enough to convict. >> how do you plea? >> no contest. hardest decision of my life. the way my lawyer explained it to me is, no contest isn't a guilty plea, it's basically saying you understand that there is a chance if you took it to trial, that you could be found guilty.

>> reporter: but afterwards, once you get out of prison, you're going to be a felon, which means, essentially, it's a guilty plea. but you don't believe you are guilty. >> no, i know i'm not guilty. i don't have to believe it, i know i'm not guilty. >> reporter: hansen continues to insist he is not the person who beat brittani marcell to the brink of death. did you take that shovel and batter brittani marcell in the head with it? >> no, i did not. >> reporter: you were not in that house? >> no, i wasn't. >> reporter: was it you who jumped through the window? >> no. >> reporter: so why didn't you fight it? >> it was hard, it just seems like everything just kind of stacked up against me. >> reporter: he says rolling the dice on up to 50 years behind bars, away from his children, was a gamble he wasn't willing to take. >> i'm most concerned about not being there for my kids. you know, i love them to death and so thinking about not seeing them and not being there for

them and to teach them wrong from right. it's scary. >> reporter: as part of his plea hansen now faces up to 18 years in prison. a judge will make the final decision next month. until then, basically a free man, that the marcell family says they can't seem to escape. just last week hansen was back in the news when brittani's sister jennifer ran into him, capturing this video at a baseball game. >> hansen's baseball night was approved by his pretrial services officer. >> he can show up anywhere and we can't do anything about it. >> reporter: detective jodi gonterman says that's about to change. she's hoping the judge will give hansen the maximum sentence. >> i think he's really convinced himself that he can fool people. we know that it's him. >> reporter: he still hasn't admitted that he beat brittani. >> no, he hasn't. why can't he give her an answer now, why he did it? >> reporter: do you still wanna

know that, why he did it? >> it may be a lie, i may not be able to believe it, but i wanna know. >> reporter: as for brittani, after a decade of fear, she's ready to move on. to people watching this, what message do you want them to come away with? >> that you can survive. i think if you have a strong mindset and to what your next goal is, i think you can get through it. >> reporter: so even though some of her memories are streaked with pain, brittani marcell says she's now ready to make more. what's your greatest hope for brittani's future? >> that she's able to get out there and face the world with strength and faith and no fear. >> and that's our program for tonight. i'm amy robach. for david and all of us here at "20/20" and abc news, have a good night and a great weekend.

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