Hundreds of students, faculty and members of the University of Illinois' Chinese community gathered outside the courthouse to show their support for Zhang and her family.
Christensen was ordered held without bond. During the nine-minute hearing, Christensen did not speak other than to acknowledge to the federal judge that he understood his rights. U.S. Magistrate Eric Long ordered Christensen to return to the court in Urbana on Wednesday to determine bond. A preliminary hearing was set for July 14, but that would be waived if a grand jury returns an indictment before then. The federal kidnapping charge carries a maximum sentence of life in prison, according to a spokeswoman for the U.S. Attorney's office.
Prosecutors said Christensen was the driver of the black Saturn Astra Zhang was seen climbing into on surveillance video. Prosecutors said that while he was under surveillance, Christensen made statements indicating his involvement and coupled with other findings, led law enforcement agents to believe she is no longer alive.
Kidnapping charges were filed Friday. Christensen is a former physics PhD candidate at the University of Illinois.
"The cops came, the FBI came, the SWAT team came and they pulled him out, and I was like, oh my goodness, because that's the guy that asked to help me. That's the guy I talked to this morning," said Porche Holmes, who lives right above Christensen.
Christensen also attended a rally held last week to call for Zhang's safe return, university officials confirmed. Christensen is seen in the crowd, wearing a black shirt and accompanied by a woman and holding her hand.
Zhang had been on her way to sign a lease at the time of her abduction, prosecutors said. Investigators believe she accepted the ride from a stranger because she was running late for that appointment.
Security cameras captured the Astra stopping by Zhang as she waited for a bus near a campus parking garage. Zhang is seen speaking to the driver for a time before getting in the car. It was the last time she was seen.
Investigators have not found her body.
The federal complaint states that Christensen had been under surveillance for nearly two weeks before his arrest. A forensic search of his phone revealing visits to a web forum called "Abduction 101," including the sub-threads "perfect abduction fantasy" and "planning a kidnapping."
CLICK HERE to read the full criminal complain
The website that hosted the "Abduction 101" forum bills itself as the most popular sexual fetish networking site on the internet, and it has faced pressure in recent months to be clearer about not tolerating behavior that could be regarded as criminal.
The federal complaint says Christensen's phone was used April 19 to visit the FetLife.com forum, including to view threads titled "Perfect abduction fantasy" and "planning a kidnapping."
FetLife describes itself as "the Social Network for the BDSM, Fetish & Kinky Community," stressing in online policy statements that it is a place for consenting adults to trade advice and images of themselves, and to arrange to meet. Established in 2008 by Canadian software developer John Baku, it now claims more than 5 million registered members.
FetLife prohibited hundreds of fetish categories this year after it was cited in several criminal cases, including one in Australia, Baku said in a February online note to members. He said that, among other things, he wanted to reduce any legal liability and risks to the wider community.
As of Sunday, however, the site still included multiple forums focused on abduction fantasies, and an "Abduction Play" group had more than 78,000 FetLife members. Among the threads still available on the site was one called "Tools of the Kidnapper" and another titled "Original kidnapping Play."
One registered user who commented in the kidnapping forum agreed that anyone who engages in such fantasies must first provide consent. But the person goes on: "I enjoy the knowledge that I COULD do anything I wanted to them. I'm well aware that once I render them helpless, their very life is in my hands."
Detailed policy guidelines on the site stress that any interaction online or in person with members must be between adults and consensual.
"FetLife's community is ... open-minded and non-judgmental," it says, adding, "Our number one priority is to create a fun and safe place for kinksters."
The Associated Press contributed to this report.