PEORIA, Ill. (WAND)- Prosecutors Friday played recordings of conversations between Brendt Christensen and his then-girlfriend, including recordings from a memorial walk held for Yingying Zhang.

FBI agents said Christensen's girlfriend had agreed to record phone calls and in-person conversations between the two. They said she wore a microphone on June 29, 2017, when she and Christensen walked home from the memorial walk. In the recordings, the person identified as Christensen tells the woman he cut Zhang's clothes off and tried to strangle her for 10 minutes. 

"She was valiant ... she fought," he said.

In the tapes, the man goes on to say he carried Yingying Zhang to the bathtub and hit her in the head with a baseball bat.

"I wasn't sure if she was dead or not, so I had a knife and I stabbed her in the neck, and she grabbed for it," he said. "I chopped her head off, and I said that was the end of it."

Later, the man identified as Christensen said Yingying Zhang was "gone forever."

"I won't tell you where she is," he said in the tape. "I won't tell anyone where she is." 

He could also be heard saying the family "won't leave until she's found" but is "going to leave empty-handed". 

In that same conversation, the man identified as Christensen compared himself to other serial killers and said he had killed 13 people including Yingying Zhang. Investigators said Thursday that had investigated those claims but turned up nothing. Away from the jury, prosecutors, defense and the judge discussed how to instruct jurors about considering those claims of other killings.

In other recordings, the man identified as Christensen alludes to FBI vehicles following him and discusses press coverage of the case.

Earlier in the day, jurors also watched video of a second FBI interview of Brendt Christensen Friday during the third day of his trial.

Christensen first gave a recorded interview to the FBI early June 15, 2017 after authorities seized his car. In that interview shown Thursday, Christensen first said he had been napping and playing video games on June 9, the day Yingying Zhang disappeared. Later, he admitted picking up a young Asian woman on campus but said he dropped her off.

>>Yingying Zhang Murder Trial: The evidence

On June 17, 2017, Christensen reached out to the FBI and offered to speak with them again. In that interview, Christensen again described picking up that woman.

"I saw that girl, just waiting," Christensen said. "She looked panicky ... I pulled up to her, asked if she needed help."

Christensen said he did not understand the woman when she said her name. He said he made a wrong turn while giving her a ride to an appointment, and she became worried.

"She started really freaking out ... grabbing her hair," Christensen said.

Christensen again said he dropped the woman off, but couldn't say where. When asked by FBI agents, he agreed to take a car ride with them to refresh his memory.

"I can try," Christensen said. "I just want to help."

Christensen also told officers that he used his large duffel bag to deliver a cat tree to his girlfriend, but he said he was not sure where he left it and thought it may have been stolen.

Christensen told investigators he was worried:

"If something was found, I would be in jail," Christensen said. "I didn't do it."

After the jury left for a mid-morning break, defense, prosecutors and the judge discussed what instructions to give jurors about considering recorded statements from Christensen suggesting he had killed 13 people. Prosecutors had said they had no corroboration that claim is true.

After the break, Special Agent Michael Carter, who took part in the interview, described continuing searches for Yingying Zhang, including a search of Allerton Park in November 2018.

Next, prosecutors called Special Agent Andrew Huckstadt, who described his role in the search of Christensen's apartment early on the morning of June 15. Huckstadt briefly discussed an interview with Christensen's then-wife, including her statement that Christensen's favorite book was American Psycho.

In the afternoon, Special Agent Huckstadt's testimony continued. 

Prosecutors are playing a series of secret phone and in-person audio recordings of conversations between Brendt Christensen and his girlfriend, after she agreed to cooperate with FBI investigators.

In one conversation, Christensen referred to a vehicle that was monitoring him.

"That truck down there is the thing following me," Christensen said. "It's obvious. They're not trying to hide it."

Huckstadt said authorities were overt in their surveillance to protect the girlfriend.

In another recording, Christensen discussed the duffel bag mentioned earlier in the trial and told his girlfriend he believed it had been stolen along with the cat tree he said he brought her. He also warned her about knowing too much about the investigation.

"It's just one of those things where, the less you know ...," Christensen said. "I don't want you caught up in this more than you have to be."

In a conversation June 27, Christensen refers to a news article about authorities' announcement that they had found the car involved in the kidnapping.

"They know it had nothing to do with me," Christensen said, adding that the announcement may have been a ploy to confuse the "real culprit."

At mid-afternoon, prosecutors showed photos of Christensen and his girlfriend at the June 29 memorial walk for Yingying Zhang. They are expected to play audio recordings of Christensen at the vigil after a mid-afternoon recess.