Transcript for 30 years after Mandy Stavik murder, arrest made with DNA match: Part 2
"Nightline" continues. It had been nearly 30 years since Mandy stavik's body was found in a river. And police why were no closer to solving the murder. But law enforcement hopes that newly obtained DNA pulled off a drinking cup used by former neighbor Tim bass, will be what finally cracks this cold case wide open. When the cup arrived, I performed a swabbing of the drinking area. I pulled up the old information to compare it to, and let's just say I was more than surprised. It was a perfect match. When Tim bass was taken into custody at his workplace, he agreed to talk to us down at the station. Just like you've seen on T.V., on those shows you've watched, I just have to advise you of this before we talk to you any further, so -- you have the right to remain silent. Tim's demeanor was flat. He didn't show a lot of emotion, tried to act like a lot of disbelief, like, "No, I didn't have anything to do with this. You've got the wrong person. I've never had any intimate relationship with her. I've never even kissed her. He's testing the waters to see whether or not they actually have his DNA versus them telling him a ruse. How'd that come about? My DNA, that you guys got? I ran a sample. I don't remember giving DNA. I'm sure in his mind, he was thinking, "I've gone through all these measures to make you guys didn't get my DNA. How did this happen? Where did I screw up? Just tell me. If you just did it sneakily, you did something weird. Well, of course I did. Okay. Well that's all you need to tell me. The point being is if I didn't have something of that nature, you wouldn't be sitting here. Right. I think once he realized we actually did have his DNA. Then, he switched, "Well, I wanted to tell you a long time ago but I just didn't trust you." I've been told not to say stuff. Ah, to hell with it. I can't do this. I can't. I trust you guys. I can't. If this bites me in the -- or this is not what I'm supposed to do, then whatever. I don't give a -- I slept with her. He completely does 180 degree turn. And at that point says, "Yes, I slept with her." Tim bass told us a story that he was having a secret relationship with Mandy. And that's how he accounted for his DNA being found. There's no way my sister would have had a relationship, a physical relationship with Tim bass. She was way, way, way out of his league, to put it bluntly. You are under arrest for the murder of Mandy stavik. The arrest occurred on December 12th, 2017. I filed charges, charged him with murder in the first degree. Shortly after Mr. Bass was arrested, I drove out to Mary stavik's home. It was her 81st birthday that day. It was. It was on my birthday that he knocked on my door. Kind of nice birthday present. She was overcome with emotion, and I think we all were. But the case wasn't closed yet. Tim's wife Gina tells police she spent that entire day with Tim. When you went to his house that day, was Tim there? Yeah. He was there the entire time? Yeah. Despite the alibi, the case still went to trial, three decades after Mandy had been found dead. To me, a trial is where I really love to be. All rise. Finally getting underway was such a relief to me. The evidence will show that Mandy was abducted, and we can tell you where it happened. My theory is, is Mandy set out to do her five mile run with the dog. About a quarter of a mile from her house where there's a wooded area, it's very secluded and that's where Tim is waiting for her to run by. He grabs her, gets her in the car. She was taken approximately six miles south to an isolated location where she was raped. I think she tried to get away by running away, naked, wearing only her shoes and socks. He caught with her and hit her in the head and knocked her out. Either struck in the head or pushed into a tree and then she was placed in the river where she was drowned. Timothy bass's defense attorneys were trying to say that just because his DNA matched that found in Mandy's body, it doesn't necessarily mean that he killed her. Did you ever see her with Tim bass? No. One of the things I did with the witnesses I called, I asked each of them if they had ever seen Mandy with Tim bass. No. Never. We wouldn't hang out with him at all, ever. The main point was they didn't know each other, they didn't hang out together, they didn't run in the same circles. The state would next call Gina Malone. Were you previously married to the defendant Tim bass? Yes. I filed for divorce. I only saw him one time in jail. By the time, Gina bass walked into that courtroom, there's a lot of change in her life. Gina came forward and said that her alibi that she originally told us was false. I believe Gina was providing his alibi to protect herself and once she felt secure and safe herself, then she could actually tell the truth. After the police came, he told me that I need to lie and say that I was with him that day. And he said, "Otherwise, I'm gonna go to prison." I felt like I just have to agree with everything he's saying because if I don't, I could be next. I wasn't a strong person back then. I just was very weak. But I should've gone with my gut instinct. The day the verdict was read, the courtroom was packed. All rise. I understand that the jury has reached its verdict. We have. We the jury find the defendant Timothy Forrest bass guilty of the crime of murder in count one. Tim bass was sentenced to nearly 27 years in prison. I thought, "Hooray. The guy's going to get what's coming to him." I give Dave Mceachran a lot of credit. He is something. And bless his heart, he told me day one that he was not going to give up until this was finalized. And we owe him the credit. Up next, the moment Mandy stavik's mom meets the woman who helped solve the case.
This transcript has been automatically generated and may not be 100% accurate.