Transcript for Suspect's mom says no evidence her son was inside home during shovel attack: Part 5
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.
This transcript has been automatically generated and may not be 100% accurate.