The girlfriend of a man accused of killing a Chinese graduate student wore a wire for two weeks to help police catch him, it has been revealed.
Brendt Christensen, 28, is accused of kidnapping and killing scholar Yingying Zhang after allegedly kidnapping her on June 9 last year.
She vanished after telling friends she was going to sign an apartment lease near the University of Illinois where she was studying.
For weeks, police suspected Christensen but had no evidence to arrest him.
In court on Tuesday, his lawyers however revealed that his girlfriend, who he was filmed attending a vigil to help find Zhang after she went missing, wore a wire to record hours of their conversations in the weeks which followed her disappearance.
Among the conversations was one in which Christensen allegedly confessed to the killing and described how Zhang, 26, fought back when he attacked her.
He is also said to have described his 'perfect victim' by pointing them out in the crowd at the vigil where he was filmed walking hand in hand with the woman.
Christensen's lawyers did not reveal how the FBI contacted his girlfriend.
They pleaded with a judge to not allow the taped conversations into evidence at his trial, claiming the girlfriend was pressured into wearing the wire because she feared charges herself.
On one occasion, she was apparently so nervous about wearing it in front of him that she fainted, his lawyers claimed.
Christensen also allegedly tried to lure another student into his car on the day Zhang went missing.
That student told investigators that he pretended he was a cop to try to get her into his vehicle. She identified him through photographs shown to her by police.
Zhang's body has never been found. Her death shocked her relatives in China who believed she would be safe in the small campus town where she spent most of her time.
Christensen is also married. It is not clear if he and his wife, who has not been identified, are estranged.
His lawyers are asking for his trial to be moved to Chicago, claiming the case was so high profile that he has no chance of a fair trial.
Zhang's family traveled to the US from her native China after she disappeared.
Her grieving father was still in the country in November last year after authorities said she was presumed dead.
He walked the route to her apartment from campus every day, he said, because it gave him comfort.
At the time, her mother said: 'We don't know where she is, and I don't know how to spend the rest of my life without my daughter.
'I can't really sleep well at night. I often dream of my daughter, and she's right there with me.
'I want to ask the mother of the suspect, please talk to her son and ask him what he did to my daughter. Where is she now? I want to know the answer.'
The FBI previously said they had obtained 'audio surveillance' from the vigil which Christensen attended.
They did not disclose that it was recorded by his girlfriend.
He is charged with kidnapping resulting in death, which carries the potential of the death penalty on conviction.