CHICAGO (CBS) — The federal death penalty trial of a former University of Illinois student accused of killing a visiting Chinese scholar continues on Monday.

Brendt Christensen’s defense attorneys are expected to continue their cross-examination of an FBI agent, following explosive recordings of Christensen bragging about killing Yingying Zhang.

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Yingying Zhang (FBI)

To this day, one knows what happened to Zhang’s body, which is exactly what Christensen boasted about in conversations secretly recorded by his girlfriend, who became a paid informant for the FBI a week after Zhang was kidnaped on June 9, 2017.

“I won’t tell you where she is. I won’t tell anyone,” he said. “They will never find her.”

On Friday, an FBI agent testified Christensen’s girlfriend became a key part of the case when she agreed to make the recordings. From June 16 to June 29, 2017, she recorded nine conversations with Christensen.

Investigators said the final recording was made the day of a walk and concert in support of Zhang, an event Christensen attended with his girlfriend. The recording revealed he talked about the crime on the walk home.

“I cut her clothes off and just started doing stuff to her,” he said.

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Brendt Christensen. (Courtesy: Macon County Sheriff’s Office)

Earlier in the conversation, he said “She was resilient. I tried to choke her to death but she didn’t.”

“I choked her for what must have been 10 minutes. Then I released her. Her breath. I couldn’t believe she was still alive,” he added.

Christensen said he hit Zhang in the head with a bat and stabbed her, and she was still alive, so he decapitated her.

He told his girlfriend he had been wanting to talk to someone about the murder, and he wanted to kill more people.

“I still want to do it,” he said. “It’s my legacy.”

Monday will be the fourth day of testimony in the trial. Prosecutors have said they could wrap up their case this week.

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Defense attorneys have conceded Christensen killed Zhang, but have disputed some details about how and why he did it, and are seeking to spare him the death penalty.