CHICAGO, June 17 (Xinhua) -- The trial of Brendt Christensen, who is accused of kidnapping and killing Chinese visiting scholar Zhang Yingying in 2017, continued Monday, with a FBI agent disclosing the defendant's digital life before his arrest.

The fourth day of testimony for the trial started Monday morning at a federal courthouse in Peoria, U.S. state of Illinois, after a secretly recorded audio was played in the court.

The recording of Christensen was made by his girlfriend when she wore a wire for the FBI as the two attended a campus vigil for Zhang on June 29, 2017.

Christensen said in the recording how he kidnapped, tortured and killed Zhang. He also said Zhang was his 13th victim.

FBI senior forensic examiner William O'Sullivan detailed that Christensen googled serial killers by victim count, downloaded a research paper on human decomposition and possessed multiple photos of naked women who were bound and gagged that he'd found online weeks before Zhang went missing.

The defendant visited Fet Life pages on perfection abduction fantasies, and looked up knife sharpening on websites. He also did a Google search for "sodium hypochlorite," or bleach.

Prosecutors believe they'll wrap up their witness testimony later this week. Christensen's attorneys said they have at least two witnesses they want to call, including Christensen's wife.

One of Christensen's attorneys admitted in the court that the defendant indeed kidnapped and killed Zhang. However, Christensen is not changing his no-guilty plea.

The defense said they will make an effort to spare Christensen the death penalty.

Zhang, a 26-year-old visiting Chinese scholar at the University of Illinois at Urbana-Champaign (UIUC), went missing on June 9, 2017, after getting into a black Saturn Astra about five blocks from where she got off a bus on her way to an apartment complex to sign a lease.

Police arrested Christensen on June 30, 2017, who was a former UIUC doctoral student, and charged him with the kidnapping, torturing and killing of Zhang.