When Brendt Christensen turned up to a vigil for the woman he is now accused of abducting, the Federal Bureau of Investigation was listening.
Because after University of Illinois scholar Yingying Zhang was last seen stepping into a black hatchback on June 9, their investigations quickly led them to suspect that Christensen had kidnapped her, reports news.com.au.
So, about a week after her disappearance, the FBI began continuous surveillance of its 28-year-old suspect, secretly recording conversations and watching his every move.
Officers were listening in when he joined Zhang's family and hundreds of others on June 29 to march through the university campus, two-and-a-half-hours' drive south of Chicago, as part of a campaign to find her.
During the vigil, he was caught talking "about the characteristics of the ideal victim" and pointed out people who fitted that description, prosecutors said during a bail hearing earlier this week.
He allegedly told another person on the tape about how Zhang "fought and resisted" while he abducted her.
He was also allegedly heard making a threat to another person who went on to provide incriminating evidence to authorities.
The FBI alleged that its audio recording also caught him explaining how he kidnapped Zhang.
"Christensen stated that he brought YZ [Zhang] back to his apartment and otherwise held her in his apartment against her will," Special Agent Anthony Manganaro said in his statement.
Neither prosecutors nor the FBI has explained who Christensen was talking to in the tape.
Police arrested Christensen the day after the vigil and charged him with kidnapping Zhang, 26, an agricultural sciences scholar visiting the university from China.
While Christensen has not been accused of killing Zhang, authorities say that due to the evidence they have gathered they believe she is dead.
Zhang disappeared on a sunny June 9 afternoon when she left home to sign the lease on a new apartment. Except she never made it to her appointment.
CCTV caught her as she got off a bus in the Illinois city of Urbana. She tried to flag down another bus but it failed to stop.
A black Saturn Astra drove past the clearly distressed woman and then circled back and stopped.
Zhang was seen to talk to the driver for about a minute before she got into the front passenger's seat and the car drove off.
She hasn't been seen in public since.
According to an FBI affidavit, after initially telling investigators that he was at home playing video games all day, Christensen later admitted to picking up an Asian woman in his black Saturn Astra that afternoon.
But he said that he dropped her off not far from where he picked her up after she became distressed.
However, investigators noted that the car's front passenger door "appeared to have been cleaned to a more diligent extent than the other doors", suggesting he had tried to conceal evidence that she had been in the vehicle.
Christensen, who has a master's degree in physics at the University of Illinois, was described by lecturers as a smart but pretty normal guy. He is unemployed and married without kids.
Police say he researched how to abduct someone two months before Zhang went missing. When searching his phone, officers found that he had visited the website FetLife.com, which hosted a forum called "Abduction 101", including sub-threads on the "perfect abduction fantasy" and "planning a kidnapping".
FetLife describes itself as "the social network for the BDSM [bondage, dominance, sadism and masochism], fetish and kinky community", where users can remain anonymous.
Christensen's lawyer Tom Bruno said he did not think a "negative inference" could be made from his client marching in the vigil for Zhang.
He said Christensen was entitled to the "presumption of innocence".
Magistrate Eric Long refused bail and ordered that Christenson stay in jail until trial.