Image copyright Police handout Image caption Brendt Christensen, a physics graduate, changed his story about picking up the missing woman, say investigators

A former PhD student has appeared in court in the US state of Illinois charged with kidnapping a Chinese student, who remains missing.

Hundreds of people gathered outside court in the city of Urbana as Brendt Christensen, 28, was arraigned in a nine-minute hearing on Monday morning.

He was arrested on Friday in connection with last month's disappearance of Yingying Zhang.

Ms Zhang, 26, has not been found, and authorities say she is presumed dead.

Surveillance video filmed her climbing into a black Saturn Astra car on 9 June at the University of Illinois campus. Investigators say Mr Christensen was behind the wheel.

Image copyright FBI Image caption Yingying Zhang was late to sign a lease for an apartment when she accepted a lift

The video shows Ms Zhang - who had been in the US barely two months and was late to sign a lease for an apartment - speaking to the driver before getting into the car.

The Champaign News-Gazette reports that Mr Christensen attended a rally held last Thursday evening in support of Ms Zhang on the University of Illinois campus in Urbana.

He was under police surveillance at the time.

According to court documents, there were only 18 four-door Saturn Astras - like the one captured on security camera footage - registered in Champaign County, Illinois. One of the vehicles belonged to Mr Christensen.

Image copyright Imgur Image caption A photo shared on social media shows a man (centre) resembling Brent Christensen at a campus walk to raise awareness of Ms Zhang's disappearance

Investigators went to his apartment and found a Saturn Astra parked outside with a sunroof and a cracked front hubcap, like the car in the footage.

Court documents say investigators also noticed the front passenger door of Mr Christensen's car "appeared to have been cleaned to a more diligent extent than the other vehicle doors".

FBI special agent Anthony Manganaro said in the criminal complaint: "I believe that this type of action may be indicative of an attempt to conceal or destroy evidence."

When authorities interviewed Mr Christensen on 12 June he initially said he could not remember his whereabouts on the afternoon of the suspected kidnapping. He later said he must have been sleeping or playing video games at his apartment.

Image copyright FBI Image caption Ms Zhang was planning to seek her PhD, the University of Illinois said

When investigators interviewed him again on 15 June, the FBI agent wrote: "Christensen admitted to driving around the UI [University of Illinois] campus when he observed an Asian female with a backpack standing at a corner appearing distressed.

"Christensen claimed that he drove up to the Asian female, who relayed she was late to an appointment. Christensen stated that he offered the Asian female a ride."

The suspect told investigators the woman panicked after he made a wrong turn and so he let her out of the car a few blocks from where he had picked her up.

However, court documents say: "Christensen was captured on audio recording while under law enforcement surveillance explaining how he kidnapped YZ [Yingying Zhang]".

Image copyright FBI

"Christensen stated that he brought YZ back to his apartment, and otherwise held her in his apartment against her will."

It is not clear how investigators recorded the suspect's remarks.

The FBI special agent wrote that "law enforcement does not believe YZ is still alive".

Mr Christensen was a graduate student in University of Illinois' physics department and a teaching assistant until May 2016.

Image copyright FBI Image caption Ms Zhang disappeared after getting into a man's car on 9 June

According to court documents, a search of Mr Christensen's phone revealed his visits to a sexual fetish networking website called FetLife.com.

On 19 April, his phone was used to view a forum called Abduction 101 and sub-threads, "perfect abduction fantasy" and "planning a kidnapping".

He maintained a profile on the dating site OkCupid in which he described himself as "married and in an open relationship".

It is not known whether Mr Christensen had any academic interactions with Ms Zhang, an agricultural sciences student.

Her disappearance triggered widespread concern in China, which has more than 300,000 students enrolled at US universities.