Transcript for DNA match links man to near-fatal shovel attack: Part 4
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 individuals 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 a violent 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, totook 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
This transcript has been automatically generated and may not be 100% accurate.