A photo of Zhang Yingying released by the police.

CHICAGO - The FBI is investigating the disappearance of a visiting Chinese scholar from a central Illinois university town as a kidnapping as her whereabouts have remained unknown since Friday.

Zhang Yingying, 26, was last seen on June 9 near the north end of the campus of the University of Illinois at Urbana-Champaign (UIUC), wearing a charcoal-colored baseball cap, a pink and white top, jeans and white tennis shoes and carrying a black backpack.

She boarded a Champaign-Urbana Mass Transit District bus and exited the West Springfield and North Mathews avenues at 1:52 pm local time (1852 GMT), police said.

The FBI has labeled the case as a kidnapping but isn't ruling out anything, said Campus police spokesman Patrick Wade.

The suspect appears to be a white male who was in a car and stopped to talk to Zhang on Friday afternoon, the FBI said.

Almost one month into a year-long appointment at the UIUC campus, Zhang's friends told police that she was heading to an apartment complex in Urbana to sign a lease.

Security camera footage on Monday released by university police showed that Zhang talked to the driver of a black Saturn Astra, about five blocks from where she got off a bus in Urbana on Friday afternoon. She entered the vehicle shortly thereafter.

Zhang has not been seen or heard from since then and attempts to contact her by phone remain unsuccessful. Authorities have asked the public to help identify that vehicle.

"We have her on video getting into a car and that's the last we see of her. That's obviously very concerning," Wade told The (Champaign) News-Gazette.

A massive search has been launched in Urbana in the US state of Illinois since Zhang's disappearance. Police are interviewing with her colleagues, checking local hospitals and coordinating with ride-hailing companies as part of their investigative efforts.

As six days have passed since Zhang vanished, it is possible that the suspect has gone to other US states, the FBI said.

In an interview with Xinhua through Wechat on Thursday, Zhang Ronggao, father of the missing visiting scholar from China's Fujian Province, expressed gratitude to all the people involved in the search and pleaded with US police to accelerate the search.

"Dad is waiting for you to come back," the father said with a choked voice.

The Chinese Consulate General in Chicago responded quickly to the incident. Deputy Consul General Yu Peng and other Chinese consuls are in close touch with UIUC officials and the US police, and are assisting relatives of Zhang to go through procedures to head to the United States.