New details in case of murdered university scholar Yingying Zhang

Brendt Christensen was found guilty of kidnapping and killing 26-year-old Zhang. His arrest and conviction were thanks in large part to audio confession obtained by Christensen’s then-girlfriend who became an FBI informant.
2:57 | 11/15/19

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Transcript for New details in case of murdered university scholar Yingying Zhang
Now to our "Gma" cover story about an international scholar would vanished at the university of Illinois and Bob woodruff traveled to China as part of a "20/20" investigation into her appearance and here with the exclusive new details. Good morning, Bob. Good morning. This is a huge tragedy for this but this case would not have been resolved unless his girlfriend came forward and went under cover. This is university of Illinois scholar ying-ying John last seen getting into this black car in June of 2017. People on campus are wondering is there a kidnapper out there? Is he going to strike again. Reporter: Authorities tracked that vehicle to its owner, a university of Illinois grad student, Brent Christianson. And this interrogation video he admits to picking up a young Asian woman but investigators release him with hopes he'd lead them to ying-ying. They throw somewhat of a hail Mary. They ask Christianson's girlfriend Tara to wear a wire for them. Reporter: The girlfriend agrees in hopes of exonerating him or helping to find Ying Ying. And he should be coming over around 5:30 so I'm turning it on. Reporter: The way the recordings progress, the first few, there was little evidence towards the crime necessarily. I have already done more than I had to for them. I want to find this girl. Reporter: But everything changes when Christianson and Tara attend a vigil. With that he spills the truth. We're at the benefit for the walk and Brent has a thing of alcohol and he is drinking while he's here. She was very frightened many times. This really impacted her, the things that were said to her. She was valiant. She was? She was. Did she fight? Uh-huh. Nobody knows what happened. Nobody will know what happened. Except for me. I am the only one. This he'll never find her. The family, you know, we are gonna leave empty-handed because to one will ever know where she is. You know, this girlfriend took a huge risk to make this happen. Without her this would not have been resolved as I said before but also the amazing thing is the local police officer, the way he was ultimately to find this suspect, you never would have expected this one. It was incredible work by them as well. You'll see these -- the story tonight on "20/20". You said they threw a hail Mary. That was chilling to hear that. Yeah, without that, you know, this would not have been -- nobody would be going to prison right now. Yeah. But he is. Bob, thank you. Thank you so much. You can see much more on a two-hour special edition of "20/20" tonight at 9:00 P.M. Eastern here on ABC.

This transcript has been automatically generated and may not be 100% accurate.

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