Boyfriend of missing Chinese exchange student: "We live in agony every day"

The family of a Chinese researcher who vanished in Illinois is appealing to the public for help. Surveillance video from June shows Yingying Zhang getting into a black Saturn car on the day she disappeared and while investigators believe she is dead, her body has not been found. 

Former graduate student Brendt Christensen has been charged with her kidnapping. Zhang was waiting at a bus stop when police say Christensen drove up and offered her a ride.

Two months later, her family is heartbroken. They still don't know what happened to her and they told CBS News' Anna Werner that they can't imagine returning to China without her.

Zhang's mother sobbed throughout the news conference where the family spoke for out for the first time since her disappearance.

"With each passing-by day, the chances of finding Yingying alive decrease," said Zhang's boyfriend, Xaolin Hou.

Zhang's mother and brother arrived in Illinois just a few days ago. Hou says they all feel helpless.

"Every day is like torture for us," he said.  "We live in agony every day."

Twenty-six-year-old Zhang came from China to study for her PhD at the University of Illinois. 

An FBI affidavit says Christensen later admitted he offered Zhang a ride and that she got in his car but claims he let her out a few blocks away. The same affidavit also says while under surveillance, he was recorded saying he brought Zhang back to his apartment and held her against her will.  He faces one count of kidnapping.

"Our client is presumed innocent. His presumption of innocence will remain with him until this case is over," said Anthony Bruno, one of Christensen's defense attorneys.

Zhang's family says they will stay until she is found. It is a Chinese tradition, Hou said, to be buried in your hometown.

"Fallen leaves returns to its roots. We need to find her and take her home," Hou said.

Christensen's trial is scheduled to start next month. His attorney told CBS News he will ask for a delay this week in order to go through all the evidence.