Transcript for How a single drop of blood and recovered memories led to a conviction in 2008 attack
Reporter: It's been said that our lives are built one memory at a time. But at the center of this story is a memory that was lost. Was it scary not being able to remember anything? It was scary after they told me what had happened. Reporter: Here in Albuquerque, New Mexico, in 2008, life for 17-year-old Britney Marcel was as carefree as the sky over the city's annual hot air balloon fiesta. She lived in this house with her single mother, Diane. One of seven siblings. It was joyful. A beautiful family. A lovely life. I couldn't have asked for anything better. Reporter: On September 11th, 2008, the unfathomable happened. It was a Thursday. Britney would be out of school early. She said, do you want to meet for lunch? I said, sure. She said, let's just meet at home. Reporter: Minutes later, Diane pulls into her driveway and walks into a terrifying scene. I unlock the door. I see this guy with a shovel. And she's bleeding. I thought really she was dead. Reporter: Her daughter lying on the floor, beaten and bloodied. She was hit on the HD by a man wielding a shovel. Diane runs fntically from the house screaming and dials 911. Is your daughter breathing right now? She's breathing but moaning, she's going to lose consciousness, please, there's blood everywhere! Reporter: Britney is whisked away in an ambulance. Investigators assess the scene, trying to determine what actually happened. 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. Reporter: They find one perfectly round drop of blood. A calling card inadvertently left behind by whoever did this. Britney Marcel is now 27 years old and says when she came to at the hospital, she had no memory of that terrifying day. I thought, you know, I was in a bad car accident. They're like, that's far from what happened to you. Reporter: Meanwhile, detectives still have that single drop of blood. They run it through the national DNA database, hoping for a match. But no luck. It was pretty obvious that unless Britney Marcel recovered memories of the attackers, that the one and only piece of evidence was a single blood drop. Reporter: Five years go by. And still no arrests. So now a frustrated Diane Marcel calls the sergeant in charge at the Albuquerque police department pleading for a fresh set of eyes. She said, let me think about it. Then she called. Said I got somebody for you. Reporter: Detective Jody gaunterman has a reputation for being relentless. Right off the bat she has an unusual suggestion. 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. Reporter: Enter forensic psychologist Dr. Leon Morris. He uss hypnosis to help patients unlock repressed traumatic memories. This is the moment Britney relives the attack. You can see her shaking. He's -- he's -- hurting me. The doctor told me she's going to probably start remembering now. Reporter: That's exactly what happened. He was a tall guy. Reporter: Detective gaunterman decides to try something else with that drop of blood. A cutting-edge DNA test. They would take a DNA profile and give us hair color, eye color, ancestry. Then they do a 3D computer-generated image what was your suspect's going to look like. Reporter: Then she gets a call from Britney. She said, Jody, I remember the name, Justin Hansen. 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. She says she met him years earlier. Do you remember any of the conversations that you had with Justin? Hey, how's your day, how was school? Reporter: That dna-generated picture comes back and it's a bombshell. I thought, wow. It looks so much like him. And they indicated a high likelihood that the suspect would have either green or hazel eyes. Fairly unique eye color. Reporter: A unique eye color that just happens to match Justin Hansen's. So at that point, Justin Hansen did jump to the top of the list. I'm investigating a case, an older case -- Reporter: This video from a police body camera. Justin Hansen is at home with his wife and three small children when detective gaunterman shows up unannounced. What did you hear that happened to Britney? I heard that somebody attacked her and she was -- could have been raped or something, I don't know much, though. When I asked Justin, had you ever been over to that house, is there any reason your DNA or blood would be in that house? He said no. Reporter: The detective asks Justin to provide a sample of his DNA. He says he wants to get back to her about that. Which he never does. We had to find it. So I met with some undercover detectives. And I asked them, can you follow this guy, get his DNA? He was at a fast food restaurant. Drinking from a cup. 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 gaunterman walked into a meeting with the crime lab analyst. She handed me a folder. I hope opened it up. It was Justin Hansen's photo. And she put "Match" on it. Reporter: Detectives now have enough evidence to arrest Justin. Detective gaunterman is at the station to meet Hansen. She tells him he's facing a laundry list of charges, including attempted first-degree murder. And he's looking at the possibility of more than 50 years behind bars. And she has something else to say. You did this to her. I didn't do it. You did this to her. I didn't do that. You can deny it all you want, I know it's you now. Reporter: What eludes prosecutors is a clear motive. The closest they could come up with is that Justin's number was found stored on Britney's phone. Even though there weren't any recent calls or texts or anything like that, the theory would have been that Justin Hansen was essentially stalking Britney, even if nobody really knew it. Reporter: There's no actual evidence to support that theory. Justin's mother, Doreen shoemaker, is adamant that her son is innocent. 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. Reporter: She believes additional testing on those weapons could exonerate her son. But that can never be done. Because back in 2015, a clerical error led to the destruction of almost all the evidence in the case. Prosecutor waymyer believes the 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: Hansen pleads no contest to attempted murder and aggravated burglary. The way my lawyer explained, no contest isn't a guilty plea, it's just basically saying that you understand that there is a chance if you took it to trial that you could be found guilty. But afterwards, once you get out of prison, you can be a felon. You're going to be a felon which means essentially it's a guilty plea. But you don't believe that you are guilty? I know I'm not guilty. Reporter: He says rolling the dice on up to 50 years behind bars, away from his children, is a gamble he just wasn't willing to take. I'm most concerned about not being there for my kids. I love them to death. Reporter: As part of his plea, Hansen now faces up to 18 years in prison. A judge will make the final decision next month. As for Britney, she's ready to move on. To people watching this, what message do you want them to come away with? They can survive. I think if you have a strong mindset into your next goal, I think you can get through it. Reporter: For "Nightline," I'm Matt Gutman in Albuquerque, New Mexico.
This transcript has been automatically generated and may not be 100% accurate.