A former University of Illinois student on trial in the kidnapping and slaying of Chinese scholar Zhang Yingying researched and chatted online about abduction fantasies before Zhang's disappearance, according to testimony Monday in the capital murder case.

"I want to test my limits and experience everything," Brendt Christensen wrote in his profile on a website named Fet Life – a social media website for adults with alternative sexual interests.

He also repeatedly read posts on the site about kidnapping and rape fantasies with titles like "abduction play", "perfect abduction fantasy" and "Abduction 101".

William O'Sullivan, a senior computer forensic examiner at the FBI, who reviewed Christensen's computer activity from February 2017 to June 30, 2017, presented the defendant's extensive online presence to a US District Court jury in Peoria, Illinois, during testimony Monday.

In a recording played for jurors Friday, Christensen tells his then-girlfriend, who was wearing a wire for the FBI, that Zhang was his 13th victim.

Earlier Monday, however, FBI Special Agent Andrew Huckstadt testified that "we've said multiple times" that evidence has not been recovered linking Christensen with other murders, according to the Chicago Tribune.

"However, that's not the same as saying it's completely impossible," Huckstadt said. "We're continuing to investigate, and that's as far as we can go at this time."

According to O'Sullivan, Christensen searched for "serial killer list America", "flogging" and about decomposing bodies. The FBI also found files with downloaded photos of bound and gagged women on the defendant's computer.

Christensen exchanged messages with another Fet Life user on April 22, 2017, about planning a sexual encounter that involved abduction.

According to data the FBI extracted from his phone, Christensen told the other user he would break into a potential victim's house and be waiting for her, and then would jump at her when she least expected it, binding her, gagging her and then putting her in a duffel bag and into his car, according to the Chicago Tribune.

He then did a Google search for "Champaign knife sharpening" the following day.

Christensen also said "I don't care how I am remembered, just that I am," in a text-message exchange with his then-girlfriend in late May – approximately two weeks before Zhang's disappearance, according to testimony.

"Fading into nothingness is not an option," he added in another text message. "I would rather destroy humanity than let that happen."

The 29-year-old suspect never revealed in any recording what he did with Zhang's body, which has not been found since she went missing on June 9, 2017.

In one of the tapes, Christensen vowed to never tell anyone what he did with Zhang's body and said it will never be found.

In a motion Sunday, Christensen's lawyers asked the court to issue an order "directing the University of Illinois (UI) to show cause why it should not be held in contempt", according to The News-Gazette.

The two sides appear to be in a standoff with the university regarding access to records of Christensen's visit to the UI Counseling Center. He told counselors that he had suicidal and homicidal thoughts during a visit in March 2017.

The university objected to a defense subpoena issued on May 30, seeking "any written communication, note, document, or other recording concerning the assessment and treatment" of Christensen at the counseling center.

On Monday morning, US District Court Judge James Shadid said he'd consider the defense's motion to make UI show cause why it's not in contempt for not responding to the subpoena from Christensen's attorneys.

Last Wednesday, the defense team made a surprise move by admitting that Christensen was responsible for Zhang's death.

"Brendt Christensen killed Yingying Zhang, and nothing we say or do during this phase of the trial is intended to sidestep or deny that Brendt Christensen was responsible for the death of Yingying Zhang," said defense attorney George Taseff in his opening statement, adding that Christensen was "on trial for his life".

The defense team, however, still entered a not guilty plea for Christensen. They are hoping to spare the former physics graduate student a federal death penalty by raising doubt on some details and evidence during the sentencing phase.

Prosecutors said Monday that they expect to conclude their presentation by the end of the week.

The ex-girlfriend of Christensen is expected to testify this week about her recording of conversations with him, according to the Tribune.