PEORIA, Ill. (WAND)- Crime scene investigators found three stains that could be blood on a mattress in Brendt Christensen's apartment, the investigators testified Monday. 

During the search on June 15, 2017, the investigators also found a baseball bat and restraints in Christensen's bedroom; when they sprayed the bat with luminol, it showed signs of blood or another substance, testified Crime Scene Investigator December Melville. 

During cross-examination by Christensen's attorney, Melville said other substances could produce the same result. 

Earlier in the day, attorneys for Brendt Christensen tried to cast doubt on the truth of Christensen's graphic descriptions of Yingying Zhang's killing captured in FBI recordings.

In those recordings made with the help of Christensen's then-girlfriend in June 2017, Christensen said he raped Yingying Zhang, tried to strangle her, hit her with a baseball bat in his bathroom, stabbed her and cut off her head.

Christensen's team began cross-examining FBI Agent Andrew Huckstadt whether investigators had been able to corroborate Christensen's claim on the recordings that he had killed 12 others people in Wisconsin, his home state.

"We have not been able to corroborate," Huckstadt said.

Christensen's attorney continued by pointing out the site where he described much of the attack happening.

"Zero DNA results matching Mrs. Zhang in the bathroom," attorney Elisabeth Pollock said.

"Correct," Huckstadt said. "Extensive cleaning done in the bathroom."

Christensen's defense attorneys also pointed to his girlfriend's remarks on the recordings that Christensen was drinking and slurring his words. They also showed news video of Christensen at a memorial walk for Yingying Zhang drinking out of a tumbler.

Later in the morning, prosecutors called FBI forensic examiner William O'Sullivan to the stand. Sullivan described forensic analysis done on Christensen's phone and computers.

Researchers found Christensen had downloaded a document called "A Critical Analysis of Research related to the Criminal Mind of Serial Killers," published by the Graduate College at the University of Wisconsin-Stout.

They also found that he had downloaded an article on human decomposition. Although the document had been deleted, photographs from the document showing decomposing bodies remained on the computer.

Forensic researchers also found photographs showing women bound and gagged.

O'Sullivan also compiled a record of text messages and web histories relevant to the search for Yingying Zhang. On June 9, 2017, the day of Yingying Zhang's disappearance, someone opened Christensen's Google Chrome at 6:31.

Christensen texted his wife and exchanged texts with his girlfriend between 11:30 and 11:48 a.m., investigators found. Between 1:11 p.m. and 2:37 p.m., Christensen received four texts from his wife but did not respond until 3:30 p.m. Investigators say Yingying Zhang got into a Black Saturn Astra shortly after 2 p.m. Researchers found continued activity on Christensen's electronics until late that night.

On June 10, Christensen logged on to Facebook around 7:47 a.m., investigators found. His phone was unplugged at 12:48 p.m., they said. At 7:28, Christensen's account viewed a web page about the missing student.

In the following days, Christensen's accounts viewed numerous news stories and police updates about the scholar's disappearance.

In her cross-examination of O'Sullivan, Pollock suggested benign reasons for some of Christensen's downloads: that he was taking a university course in deviance at the time he downloaded the paper on serial killers and that he and his wife had been discussing decomposition during a road trip.

She also pointed out that researchers like O'Sullivan often encounter pornography on computers and other devices:

"On Brendt's computer, there was a relatively little amount of pornography, compared to what you've seen?" she asked.

"Correct," he answered.

More generally, Pollock suggested the digital material introduced by prosecutors was just a small fraction of the material found on Christensen's devices. She also pointed to text exchanges between Christensen and his girlfriend about Christensen's drinking.

Later in the afternoon, prosecutors called another FBI agent and a University of Illinois Police detective to the stand and played clips from the previously-shown interviews of Christensen in which each man was involved.

Among the points they highlighted were Christensen's statements that the woman he picked up on June 9 spoke broken English. Christensen's defense team has taken issue with assessments of Yingying Zhang's proficiency in the language.

Before a break Monday, Christensen's attorneys said they plan to call two witnesses, including Christensen's ex-wife.

Christensen's attorneys also filed a motion Sunday arguing that the University of Illinois failed to comply with a subpoena for records related to Christensen's visit to a U of I counseling center. You can find a PDF of that motion on this page.