Chilling testimony in the case against Brendt Christensen, the man accused of kidnapping and killing Chinese scholar Zhang Yingying two years ago. The drama on Thursday surrounded the defendant's former girlfriend. CGTN's Dan Williams has the latest from Peoria in the US state of Illinois.

With U.S. federal prosecutors close to wrapping up their case in the Brendt Christensen trial, they called for their key witness.

Less than a week after Chinese scholar Zhang Yingying disappeared, Terra Bullis, Christensen's former girlfriend, agreed to wear a wire for FBI investigators.

With Zhang's family present in the court, those recordings were played to the jury. A critical recording occurred on July 29th, at a vigil for Zhang, 20 days after she had disappeared. Christensen had wanted to attend.

On the tape, Christensen is heard saying that he just wanted to see how many people were at the event "for him".

"They have no idea what happened," he says. "I'm the only one."

DAN WILLIAMS PEORIA, ILLINOIS "Bullis told the court that at one point during the vigil, Christensen took her phone and typed four lines of text. Deleting each one as he went."

They read:

"It was me."

"She was number 13."

"She is gone."

"Forever."

On the recordings, he described in detail how he kidnapped, raped and then killed Zhang Yingying. How Zhang had fought more than anyone else. "Stronger than any victims I ever had," Christensen is heard saying. He describes how he "broke her head open" with a baseball bat but that Zhang still fought back. He then says, he cut off her head. He is heard on the recording laughing as he says: "None of this zombie. She was done." He says on the tape that he will never reveal where Zhang's body is.

The testimony followed a video recording played in court - of Christensen being interviewed by FBI agents just a few days after Zhang's disappearance. That recording reveals how Christensen changed his story.

"Maybe I am getting my days mixed up."

"OK."

"I thought I drove around on Saturday. I did pick a girl up. I don't remember where."

"Where did you take her Brendt." 

"We need to find Yingying."

"Tonight."

Earlier, an FBI forensic examiner told the court that DNA found in Christensen's apartment was an overwhelming match to that belonging to Zhang. Jury deliberations are expected to start next week.

Dan Williams, CGTN, Peoria, Illinois.