University of Illinois scholar Yingying Zhang was last seen at a campus bus stop on June 9, 2017, getting into a black Saturn Astra.
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As Zhang's friends and campus police searched for the visiting Chinese student, Sgt. James Carter with U of I police, began searching through security footage for clues that could help trace the vehicle.
"It was too pixelated to actually see the license plate so at that point I started looking for something distinctive about that vehicle," Sgt. Carter told the I-Team on Thursday in an exclusive interview. "I kept looking for about five or 10 minutes and I found that there was a defect on the passenger side front tire, front hubcap actually."
That observation allowed police to search through the 18 Saturn Astra's registered in the Champaign-Urbana area, including one registered four miles down the same road as the campus police department. It was registered to Christensen.
For Carter, the find came on his day off, when he came to work and volunteered to help in the investigation. It was his first actual missing persons case.
"When I was a detective, I kind of took pride in kind of digging through video and looking for really small things that perhaps most people won't look for so I always looked for the needle in the haystack. That was kind of my thing, and sometimes I would get kind of tunnel vision on finding stuff like that, but it was effective," Carter said during the I-Team's visit in Urbana.
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Christensen was the needle in U of I's haystack.
Campus police detective Eric Stiverson was on the team that went to Christensen's apartment.
"It was late in the evening. We went to his residence, we asked if he would come speak to us at the FBI facility locally here in Champaign, which he agreed to do," Stiverson recalled. "We started interrogating or interviewing him. He thought it was an interview but it was actually an interrogation. We were pretty sure we had our guy."
Stiverson, seen on video in the interrogation room with Christensen, told the I-Team that it became obvious during the questioning that he was lying.
"It was everything from his leg tittering to his eyes fluttering back-and-forth like at a high rate of speed to his the way his voice would change. He got real dry mouth... and he even broke out in hives, where you could see the blotches all over his real pale skin and you could see these red blotches come out of him," the detective told us. "I realized during the interview that he had killed her."
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Even after Christensen was charged with Zhang's disappearance, authorities have never found the woman's remains.
RELATED: Murdered U of I scholar Yingying Zhang's remains disposed of in trash, may never be recovered
After being convicted by a federal jury in Peoria, Christensen was sentenced two months ago to life in prison with no chance for parole or early release.
The FBI is still coordinating a search for her body, or what may be left of it.
RELATED: U of I kidnap victim Yingying Zhang's body could be in downstate landfill
For U of I campus police, the matter was a wake-up call.
"This case was an eye-opener that tragedy can happen anywhere and safety is something that we all have to work together to try and ensure the likelihood that people are safe," said Sgt. Carter. "I think it kind of woke people up a little bit that bad things can happen and seemingly safe places."
Det. Stiverson suggested that in tragedy, the best side of law enforcement can be seen.
"We keep and maintain a good relationship, working relationship, so that when something like this happens we can pull all of our resources together and fly under one flag and work together and come to a resolution" he said.