PEORIA, Ill. — The ex-girlfriend of a man accused of kidnapping and murdering visiting University of Illinois Chinese scholar Yingying Zhang, 26, said he was “happy” and laughing on the day they went to the woman’s vigil.

Terra Bullis, 26, continued testimony Thursday in federal court in Peoria. She wore a wire for the FBI to record conversations between her and 29-year-old Brendt Christensen after federal agents began investigating following Zhang’s disappearance on June 9, 2017.

One of the places where Bullis recorded Christensen was the June 29, 2017 vigil for Zhang’s family, friends and supporters amid the search for her. Christensen was seen in a photo attending the vigil.

Bullis said her then-boyfriend was laughing that day as he described brutally raping and beheading Zhang. When Christensen saw the crowed at the Urbana vigil, she said he told her, “They’re all here for me.”

He later typed four lines into her cellphone that read, “It was me. She was number 13. She is gone forever.”

At one point during the recordings, Bullis’ heart was beating so loudly, it can be heard on the tape.

Christensen faces the death penalty after being accused of abducting, raping and murdering Zhang, who was studying at the Urbana-Champaign campus when she went missing. Zhang was waiting for a bus when prosecutors said Christensen lured her into his car. They said he then took her to his apartment, raped, choked and stabbed her before beating her to death with a baseball bat and decapitating her. Her body has never been found.

Zhang’s family, who lives in China, traveled to campus when she went missing. They were at the vigil and on that day, Bullis recorded Christensen saying, “They are going home empty-handed, because no one will ever know where she is.”

Christensen described how the 26-year-old scholar fought for her life. He said she was “valiant,” and said he choked her for 10 minutes.

“She was stronger than any victim I’ve ever had,” he said.

In the recordings, Christensen described to Bullis how he lured Zhang into his car. At one point, he makes the claim that Zhang was his 13th victim.

Prior to Thursday’s testimony, prosecutors said Christensen searched the internet for basic instructions to committing murder before the woman vanished, which one expert on serial killers said may be a sign that his boast to have killed 12 other people is false.

“Do you think you might be the next successful serial killer?” Bullis asks him in one conversation. He answers: “I already am.” He calls Zhang victim “No. 13” and says he’s been killing since around 2001 when he was 19.

Bullis testified Wednesday and described her relationship with Christensen and mentioned their shared interest in bondage and fetish websites. She said there was somebody on the website he was talking to about an abduction fantasy.

Bullis said she decided to wear a wire for the FBI because she was emotionally attached to Christensen and wanted to know what was happening. Bullis wore the wire over several weeks, once becoming so nervous recording her boyfriend-turned-suspect that she fainted.

Bullis and Christensen were dating while Christensen was married. He and his wife had an open relationship.

Christensen has pleaded not guilty to kidnapping resulting in death, though his lawyer told jurors during opening statements he did kill Zhang.

For someone who purports to be a seasoned killer going back to his days as an undergraduate in physics in Wisconsin, Christensen devoted a lot of time researching the topic leading up to Zhang’s disappearance.

He downloaded an article, “Beyond the Grave —Understanding Human Decomposition,” and a paper on “The Criminal Mind of Serial Killers,” and he visited an “abduction 101” fetish forum.

“The contents of Christensen’s internet search history demonstrate his lack of knowledge of basic things that proficient serial killers with high body counts would know,” Enzo Yaksic, a Boston-based researcher who has studied serial killers for over 15 years, said.

But Christensen does share some traits with convicted serial killers.

He targeted a stranger, prosecutors said. Sexual fantasies underpinned his desire to kill and he idolized serial killers in history, especially Ted Bundy. A recent study co-authored by Yaksic noted serial killers often share a fondness for violent fictional characters. Christensen’s favorite novel, prosecutors say, was “American Psycho,” about a young professional who kills at night.

A 2015 FBI report on serial killers said 70% percent were highly stressed before they began killing. In 2016, Christensen marriage was unraveling. The once straight-A student began getting Fs in all his classes and he abandoned his quest for a Ph.D.

He also shared a longing for infamy, texting two weeks before Zhang went missing that, “I don’t care how I will be remembered, just that I am.”

Choking is also a marker for some serial killers, said Yaksic, because it satisfies their craving for control.

Killers have been known to exaggerate their number of victims, possibly in a bid to become yet more notorious. Before he was executed in 1989, Bundy claimed to have killed 100 women after saying it was 30.

The claims often can’t be proven or disproven, so it’s likely no one will ever be able to say with complete certainly Christensen is lying.

Prosecutors dangled the possibility Christensen killed before during openings last week. Under most circumstances, mention of previous, unproven crimes would lead to a mistrial. But prosecutors seem to want to illustrate, not that Christensen actually killed others, but that homicidal fantasies motivated his killing of Zhang.

Agent Andrew Huckstadt told jurors this week that the FBI continues to investigate Christensen’s claims. He said that saying they’ve been unable to corroborate them is “not the same as saying it’s completely impossible.”

The investigation likely involves checking if Christensen’s DNA matches of DNA found at the scenes of unsolved homicides in Illinois and Wisconsin. He grew up in Stevens Point, Wisconsin, got his degree at the University of Wisconsin-Madison before moving to the Champaign-Urbana in 2013.

Government filings say there’s evidence of at least one previous assault. They say a woman, referred as “M.D.,” told the FBI after Christensen’s arrest that she met him for a date in 2013. After meeting for coffee, she says he drove her to a cemetery, choked her and sexually assaulted her.

Defense lawyers say Christensen was in a drunken stupor when he spoke about other victims and that it isn’t true. They’ve also denied M.D.’s allegations.

Actual serial killers, explained Yaksic, demonstrated more patience than Christensen, who only happened upon Zhang and pulled up to her during the day along streets lined with surveillance cameras.

Yaksic says he thinks there’s only “a minuscule chance” Christensen killed before, categorizing him as “wannabe serial killer.”

“Wannabes,” he said, “are often compelled by a mixture of emotions and hubris … aspects of their personality that lead to their apprehension before they come close to achieving their goals.”