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PEORIA — On the fourth day of the trial of Brendt Christensen, the prosecution presented as evidence the baseball bat that he said he hit Yingying Zhang on the head with after kidnapping her.

Illinois State Police crime-scene investigator December Melville pulled the Louisville Slugger bat out of its evidence box and confirmed it was the one that glowed blue when she sprayed it with luminol — a chemical used to detect trace amounts of blood — at Christensen's apartment six days after Ms. Zhang was last seen, on June 9, 2017.

Melville used a cotton swab to take a sample of DNA for further testing.

In a wire recording by his girlfriend, Christensen said he hit Ms. Zhang in the head with the bat "as hard as (he) could," but realized he didn't get rid of all the evidence that could lead back to him.

"They have the bat I hit her head with," he said to his girlfriend.

The FBI biologist who tested the DNA is expected to testify Wednesday, but in the prosecution's opening statement, Assistant U.S. Attorney Eugene Miller said Ms. Zhang's DNA was found on the bat, as well as on three stains from Christensen's mattress.

Pictures of those stains were shown Monday to the jurors, with a small reddish stain in the middle of the bed next to a more faint but larger stain, and a larger stain at the head of the bed.

Melville also swabbed those stains, and they later tested positive for Ms. Zhang's DNA, Miller said.

Melville also found a knife in the utility room, though she said that didn't result in any useful evidence.

And she noted baking soda on the floor of the utility room and two empty boxes of Arm & Hammer baking soda in the trash can.

Online activity

The jury also heard details about Christensen's online activity from the months leading up to Ms. Zhang's disappearance until he was arrested June 30, 2017.

While senior FBI forensic examiner William O'Sullivan said no relevant location data was recovered, the first item he flagged was on Feb. 6, when Christensen downloaded an academic paper on the minds of serial killers.

Later that month, Christensen visited an article on decomposition and downloaded photos in the article of human bodies decomposing.

In April, he created an account on the fetish social network FetLife, not long after meeting his new girlfriend.

He would also Google "serial killer list America" and visited the Wikipedia page about serial killers ranked by their number of victims.

That same day, he also downloaded four images of bound and gagged women.

The jurors were briefly shown the images of the women, one bruised badly, another with duct tape over her mouth and another chained to a dirty bed.

The images were among hundreds of photos and videos placed in a hidden directory on his computer with his pornography. While it likely kept the files hidden from his wife, it was a "beacon" for the FBI, O'Sullivan said.

Consensual abduction

Later in April, after visiting a question-and-answers section of FetLife on abduction fantasies and the basics of abduction, he responded to a woman's request for a consensual abduction.

Christensen wanted to know when she was home alone, so he could break into her house and abduct her, perhaps while she was taking a shower.

He said he'd gag her, put her in a large duffel bag and take her to a motel.

He also said he was looking into a letter of consent in case the police or someone else accused him of something illicit.

But the woman had two roommates, so she didn't know when she'd be home alone.

Christensen said he'd never done an abduction before, but had done some planning.

He would later buy a 6-foot-long duffel bag days before kidnapping Ms. Zhang and claim he used the bag to transport a "cat tree" he bought at Wal-Mart for his girlfriend. He said the bag broke, so he didn't use it, and it may have been stolen.

The duffel bag hasn't been located, nor have investigators been able to find footage or records of Christensen buying the "cat tree," a toy for cats to climb on.

Knife sharpening

Later in April, Christensen searched YouTube for knife sharpening and Googled "champaign knife sharpening."

On April 28, he texted his girlfriend that he bought a bed restraint, blindfold and gag.

These purchases came up later in the day when Melville testified about what she found at Christensen's apartment. While bed restraints were found, a blindfold and gag were not, she said.

In May, Christensen texted his girlfriend that he was drinking and driving by a cop, but wasn't drunk.

"I would never do anything so dumb I go to jail, though that would be very interesting," he said.

That same day, he texted his girlfriend that "Fading into nothingness is the default for most people," but "I would rather destroy humanity than let that happen."

The day of and after

O'Sullivan then went through Christensen's digital activity the day he kidnapped Ms. Zhang.

Beginning at 6:31 a.m., he checked his Reddit messages, then Gmail.

At 11:34 a.m., he texted his wife, though the contents of those texts have been redacted.

Four minutes later, he texted his girlfriend after she told him about having casual sex the night before and that "you don't do the anything casual thing" ... "from breathing to fine dining to ... murder."

At 12:36 p.m., his phone started using cellular data, and 1:11 p.m., he received a text from his wife.

He didn't respond until 3:30 p.m., an hour and 26 minutes after he kidnapped Ms. Zhang.

At 4:53 p.m., he texted his girlfriend, "I'm exhausted."

That evening, he watched porn, and the last digital activity that night was at 1:48 a.m., when he plugged his phone in to charge.

Christensen then started using his phone the next day at 7:47 a.m., checking Facebook, and throughout the day, he looked at Reddit and News-Gazette and University of Illinois Police Department articles on the search for Ms. Zhang.

After the FBI visited him June 12, he deleted his Google Chrome web browser history at 6:04 a.m. the next day. O'Sullivan said some of that browsing history was able to be recovered.

Over the next several days, Christensen checked on updates to the search until he was arrested June 30, 2017.

Cross-examination

In her cross-examination of O'Sullivan, Assistant Federal Defender Elisabeth Pollock noted the limitations of O'Sullivan's analysis, as it only included a limited subset of Christensen's browsing history taken without context.

She asked him if he knew that the academic article on serial killers was downloaded while Christensen was taking a UI class about deviance. O'Sullivan did not.

She noted that the total time Christensen spent looking up Wikipedia pages on serial killers amounted to no more than a few minutes.

And she said finding porn on Christensen's computer hardly makes him unique.

"That appears to be the norm" in devices he's examined, O'Sullivan admitted, and agreed that only a few of Christensen's pictures were "disturbing," as Pollock put it.

She also noted that Christensen's searches about abduction fantasies in sections of FetLife indicate his unfamiliarity with the subject and that he only visited the site a handful of days.

As for the search about knife sharpening, Pollock suggested his wife asked him to take care of the kitchen knives.

On Monday, Pollock said the defense plans to call Christensen's wife as a witness. She last testified in December at a hearing to exclude evidence seized from their apartment. She testified that she was startled by the FBI agents pounding on her door late at night and claimed they searched her apartment before she gave consent.

The defense's motion failed, after U.S. District Judge Jim Shadid found the FBI agents' testimony that they searched after she gave consent more credible than Christensen's wife's.

'Aggravating factors'

While the defense has admitted Christensen killed Ms. Zhang, they're hoping to spare Christensen the death penalty by casting doubt on the prosecution's "aggravating factors," or reasons for the death penalty.

Those include:

— That Christensen committed the offense "in an especially heinous, cruel or depraved manner, in that it involved torture or serious physical abuse." The defense seems to be arguing that the "torture and serious physical abuse" Christensen describes on the tape can't be trusted since he was bragging and drunk.

— That Christensen killed Ms. Zhang "after substantial planning and premeditation." In its opening statement, the defense made it sound like Christensen decided to kill Ms. Zhang only after he had hit "rock bottom" when his wife left for the weekend with her new boyfriend.

— That Christensen "is likely to commit criminal acts of violence in the future" because of "his expressed desire to be known as a killer; and his claims of additional victims." Those two things come from the wire recording, which the defense is trying to discredit because of his drunkenness and outlandish claim of 13 victims.

— That Christensen's victim "was particularly vulnerable due to her small stature and limited ability to communicate in English." In a cross-examination last week of Ms. Zhang's professor, Kaiyu Guan, Pollock noted that part of the reason he hired Ms. Zhang was because of her English skills, even reading an email sent by her written in good English. But Guan said her English skills were simply good for an international student.

More testimony

Also Monday, Illinois State Police crime-scene investigator Timothy Lemasters testified about his search of Ms. Zhang's apartment 11 days after she was last seen.

The business card for her to contact police if she returned was still there, and the evidence tape from shortly after she disappeared hadn't been tampered with.

Lemasters said he looked for footprints that might be similar to Christensen's, but found none, and he also obtained items that might contain Ms. Zhang's DNA.

He also took several pictures, documenting her apartment as she left it, with M&Ms and tortilla chips on her living-room table, a refrigerator full of food, school notes on her whiteboard, cooked rice in her rice cooker and her journal on her dresser.

Investigators later gave Ms. Zhang's family a replica of this journal.

The final page is written in English and says, "Life is too short to be ordinary."