Suspect Brendt Christensen's comments were recorded by the FBI, which began surveillance of Christensen about a week after the June 9 abduction. It wasn't immediately clear how the recording was made.
Bryan Freres, an assistant US attorney, told a federal judge in Illinois that Christensen, 28, was also recorded explaining how he kidnapped 26-year-old Yingying Zhang. Christensen said in the recording that Zhang fought him, and that he took her back to his apartment and held her there against her will last month, according to Freres. It was not clear where that recording was made.
Freres said Christensen was also caught on an audio recording threatening someone who then provided incriminating evidence to authorities. The prosecutor said he didn't see a "combination of conditions" where Christensen was not a danger to the community. US Magistrate Judge Eric Long agreed and denied bail for Christensen, who was charged on Friday, a day after the June 29 vigil.
Long said the fact that Zhang is still missing "weighs against you." He pointed out that Christensen was seen on video driving the vehicle that Zhang climbed in.
"You were the last person to see her," the judge said.
Suspect photographed at vigil
A photo taken by CNN's Kaylee Hartung on June 29 showed Christensen was at the vigil that hundreds attended, including Zhang's family
Zhang was last seen getting into Christensen's car at a bus stop on June 9, a complaint said. Zhang, a University of Illinois Urbana-Champaign student who was researching photosynthesis and crop productivity, is presumed dead, officials said last week.
Christensen's attorneys argued that he had no criminal history, has ties to the community and should have a right to take part in his own defense.
Defense attorney Tom Bruno said that lawyers in Illinois are ethically required to keep evidence in their possession, so they would only be able to pour through evidence with Christensen in person at a Macon County jail where he is being held an hour from their offices.
"If we're given a thousand pages, we can't make photocopies and leave them for him to read through carefully. We have to sit there with him while he looks over our shoulder," Bruno said.
Christensen has not entered a plea in the case.
A series of unusual circumstances
Several factors led police to charge Christensen last Friday, said FBI agent Special Agent Anthony Manganaro.
Christensen's unusual car -- a black Saturn Astra -- had a sunroof and a cracked front passenger hubcap, like the vehicle that picked up Zhang on the day she vanished.
Christensen also changed his story about what he was doing on June 9.
He first said he couldn't remember where he was between 2 p. m. and 3 p. m. of the day of the kidnapping, the complaint said
. He later told authorities he was probably sleeping or playing video games "at his residence all day," the FBI agent wrote in the complaint.
Days later, Christensen admitted to investigators he was "driving around the UI campus when he observed an Asian female with a backpack standing at a corner appearing distressed," the complaint said.
Christensen said he drove up to the woman, who told him she was late to an appointment and offered her a ride, the complaint said.
Christiansen also told authorities he may have made a wrong turn, and the woman panicked. At this moment, he said he let his passenger out of the car a few blocks away, the criminal complaint said.
Investigators searched Christensen's phone and discovered an April 19 visit to the website FetLife -- where the user visited a forum called "Abduction 101,"
with threads called "Perfect abduction fantasy" and "planning a kidnapping," according to authorities.
Investigators sought search warrants and started their surveillance of Christensen on June 16.
The complaint also mentioned the June 29 recording, in which Christensen was heard "explaining how he kidnapped Y. Z.," or Yingying Zhang.
Former professor 'shocked' at accusation
Christensen, who is currently unemployed, was a graduate student in the University of Illinois' physics department and a teaching assistant until May, Lance Cooper, an associate head for graduate studies in the department, said.
Matt Herndon, a University of Wisconsin-Madison physics professor, said he taught Christensen in his undergraduate particle physics class during the 2011-2012 school year. In a statement, he said Christensen was "a good student," CNN affiliate WKOW reported.
Christensen later worked as an undergraduate researcher for Herndon.
"I'm shocked to learn of the crimes he is suspected of committing," Herndon said.
He said he hoped there was "some possibility that she may still be found unharmed."