Video shows investigators confronting accused killer of missing University of Illinois student

Yingying Zhang's body has not been found.

ABC News has obtained video of a former teaching assistant at the University of Illinois accused of kidnapping and killing a young Chinese visiting scholar being confronted by the FBI and university police.

Brendt Christensen, 29, who is currently standing trial in federal court, is accused of kidnapping and killing Yingying Zhang, 26, who was last seen on video entering a black Saturn Astra on June 9, 2017.

The young agriculture researcher was never seen or heard from again. Within days, investigators tracked that car back to Christensen.

In his first video recorded interview with law enforcement, Christensen says he was elsewhere that day but then abruptly changes his story -- telling investigators he did pick up a girl matching the missing Chinese scholar’s description.

In the early hours of June 15, 2017 -- soon after investigators tracked that car back to Christensen -- University of Illinois Police Detective Eric Stiverson and FBI Agent Anthony Manganaro brought him in for questioning.

For the first 10 minutes of the 55-minute interview, Christensen told investigators he was sleeping and playing video games on the day of Zhang’s disappearance.

However, he soon changed his story.

"Maybe I mixed up the dates," Christensen told investigators. "I did pick a girl up."

On the stand, Detective Stiverson testified about Christensen’s nervous demeanor, saying Christensen began “hyperventilating” when he changed his story.

“Why would you lie to the FBI?” an investigator asked in the taped interview.

“I mixed up the days...” Christensen replied.

Although he did not admit it was his car seen on the widely-spread surveillance video of Zhang getting into a black Saturn Astra on June 9, Christensen did admit in the interview to picking up a young Asian female matching the description of Zhang.

Christensen told investigators he let her out in a "residential area" shortly after she got in his car. He struggled to explain where he allegedly dropped her off, and could not provide a precise location to investigators.

By the end of the interview, Christensen asked for a lawyer.

Along with one count of kidnapping resulting in death, Christensen is charged with two counts of providing false statements to investigators.

Although he had pleaded not guilty, Christensen has now admitted to kidnapping and killing Zhang, according to his lawyer’s opening statement in court last week. The defense added that it took issues with "the way the government says the events occurred."

Monday in court:

In court on Monday, prosecutors showed jurors Christensen’s alleged digital footprint -- before and after he is accused of kidnapping and killing Zhang.

William O’Sullivan, a senior forensic examiner with the FBI’s Springfield office, testified about Christensen’s web history and numerous explicit photos investigators say they found on his desktop computer.

O'Sullivan described Christensen’s Fetlife profile, a website described in court by the defense as "the Facebook for kinky people." On the site, Christensen frequented a forum titled “Abduction 101: Questions and answer," according to O'Sullivan.

During arguments, Christensen’s defense team showed jurors a conversation between Christensen and an anonymous user.

According to them, Christensen wrote to the anonymous Fetlife user, “I’d bind you, gag you, and likely put you in a duffel bag so no one could see you."

Prosecutors have made reference to a 6-foot long "super tough heavyweight" duffel bag Christensen allegedly bought on Amazon days before Zhang's disappearance, which investigators have not yet been able to locate.

Christensen continued to tell the anonymous user, who agreed to draft a consent form, that he had planned a kidnapping numerous times in the past, but "not usually the consent part," O'Sullivan testified.

Along with the Fetlife data collected from investigators, the court was also shown graphic pictures of women bound, gagged and assaulted, all images that prosecutors allege were found on Christensen’s desktop computer — located in a hidden folder. The origin of these photos is unclear, but prosecutors say they were downloaded from the internet.

When looking at Christensen’s search history, much of which O'Sullivan testified was recovered after being deleted, the investigator also said that they found an article from February 2017 titled, "Beyond the Grave: Understanding Human Decomposition," which includes pictures of human remains.

In that same month, Christensen downloaded a research paper titled "A Critical Analysis of Research Related to the Criminal Mind of Serial Killers," O'Sullivan told jurors.

In the days leading up to the disappearance, Christensen's search history included Wikipedia pages for “serial killers,” “serial killers by the numbers" and "David Parker Ray," one of America’s most notorious serial killers, O'Sullivan testified.

Multiple text messages between Christensen and his then-girlfriend were also shown to jurors.

On the day of the alleged kidnapping, just hours before Zhang is seen getting into what investigators say was Christensen’s car, O’Sullivan says he texted his girlfriend about the casual nature of their relationship.

"You don’t do the anything casual thing," wrote Christensen. "From breathing, to fine dining... to murder."

The day after prosecutors allege Christensen killed Zhang, O’Sullivan testified that he took to the internet to monitor the search efforts for the missing student.

In addition to O'Sullivan, prosecutors called several other investigators, both with the FBI and local law enforcement, to detail the search warrants executed on Christensen’s apartment on June 15.

Multiple pictures from the apartment shown in court included stains on Christensen’s mattress, a baseball bat and leather bondage straps on his bedroom floor, discarded hair from a vacuum cleaner and a knife found in his utility closet.

Investigators who testified showed possible positive results for blood on multiple items throughout the apartment. A DNA forensic analyst is expected to testify for the state this week.

Prosecutors indicated in their opening statement that multiple DNA samples collected from Christensen’s apartment matched Zhang.

Zhang's body has not been found.

ABC News' Elle Luan contributed to this report.