The Chinese consulate in Chicago is asking for a thorough investigation and swift justice in the case of a Chinese scholar, Zhang Yingying, 26, who is feared dead after an apparent kidnapping.

"We are shocked and heartbroken about what happened to Zhang," said Hong Lei, China's consul general in Chicago, adding that the feelings are shared by all Chinese people.

Brendt Christensen, 28, was formally charged in US federal court on Friday with kidnapping Zhang more than three weeks ago.

According to an affidavit, FBI agents, who had Christensen under surveillance after identifying his vehicle, over heard him explaining that he kidnapped Zhang on June 9.

"Based on this, and other facts uncovered during the investigation of this matter, law enforcement agents believe that Ms Zhang is no longer alive," a statement by the US Department of Justice said.

The criminal complaint said Christensen had read about kidnapping on a fetish-oriented social networking website, FetLife. It said police found that the part of Christensen's car where Zhang was believed to have been sitting had been cleaned.

Hong, the consul general, visited members of Zhang's family on Saturday morning, expressing condolences and promising support.

Zhang's boyfriend, Hou Xiaolin, said the most urgent thing is to find Zhang quickly, and he expressed hope that those responsible will be given harsh punishment.

Zhang, a graduate of Peking University, was conducting research at the Department of Natural Resources and Environmental Sciences at University of Illinois at Urbana-Champaign. She was living in university housing.

She was reported missing on June 9, when she was supposed to see a prospective landlord.

Her last phone activity was at 1:30 pm that day - a call in which she told the landlord she was running late and would not be there until around 2:10, local media reported.

Zhang's father and other family members arrived at the university from China in mid-June.

Christensen, will remain in custody pending his initial court appearance in Urbana, which is scheduled for Monday at 10 am.

An associate chancellor of the university, Robin Kaler, said on Saturday that Christensen had earned a master's degree in physics from the University of Illinois in May, and that "his affiliation with the department ended that same month", according to The Associated Press. Christensen's LinkedIn profile online states that he is a PhD candidate in physics at the university and has been a graduate teaching assistant there since 2013, the AP reported.