Family of missing Chinese scholar vows to find her Yingying Zhang, 26, was kidnapped on June 9, authorities say.

 -- More than two months after a visiting Chinese scholar at the University of Illinois at Urbana-Champaign was allegedly kidnapped, authorities say she is presumed dead but her body has not been found. Her family members, who are from China, are staying in Illinois and have not given up on finding her.

Brendt Christensen, 28, of Champaign, Illinois, was arrested on June 30 and charged with the kidnapping of 26-year-old Yingying Zhang, the FBI said. Authorities claim Christensen lured her into his car in broad daylight on June 9. Surveillance video shows her getting into the passenger seat of a car that day.

Christensen has pleaded not guilty, his attorney, Evan Bruno, told ABC News.

Zhang's boyfriend, Xiaolin Hou, said at a news conference this afternoon that when Zhang's mother learned of her daughter's disappearance, "she fainted immediately."

"A lot of time after that, she could not eat or sleep. She was extremely weak," Hou said. "In these two months, we continue to encourage her that she must hurry to get well."

He added that Zhang's mother has since begun "to try her best on eating and sleeping on time, enduring great pain in her heart."

Zhang's father is also not in good health, Hou said.

"He often feels an unendurable pain in his heart," he said. "During these two months in America, he asked me the same question most every day: why there's still no update about where Ying is."

Hou spoke of the family members' difficulties during their past two months in the U.S., explaining that they are not familiar with the laws, customs and culture and are dealing with a language barrier.

But he emphasized that Zhang means the world to her parents and they do not want to return to China without her.

He also said Zhang would want to be with them, too, explaining that "this may be the last desire we can fulfill for her."

"We will never give up on her," he said. "I have no energy or time to be anxious, frustrated or angry. There's only one thought on my mind. That's to try everything I can to find her."

Zhang's family members wrote a letter to President Trump requesting that he "direct all available federal law enforcement and investigatory resources be used to find our daughter as soon as possible." The letter was read during the family's press conference today.

"As a loving father to your own children, you can understand what we are going through," the letter said. "Yingying meant the world to us."

According to a criminal complaint, during an interview at the FBI's Champaign office, Christensen "admitted to driving around the UI campus when he observed an Asian female with a backpack standing at a corner appearing distressed." Christensen then claimed that he drove up to her and when she said she was late to an appointment, he offered her a ride. He said she got into his car and tried to show him where she needed to go, the complaint said.

According to the complaint, "Christensen claimed that he believed he made a wrong turn, because the female became panicked, at which point Christensen claimed that he let her out of the vehicle in a residential area a few blocks away from where he picked her up."

But on June 29, Christensen allegedly was "captured on audio recording while under law enforcement surveillance explaining how he kidnapped" Zhang. The complaint said Christensen admitted to bringing her back to his apartment and holding her there against her will.

But Bruno, Christensen's lawyer, said: "he is legally presumed innocent." Bruno added that the defense team is "meticulously going through the mountain of evidence we have," explaining that they are "in the early stages of that long process."

"All the lawyers here at this firm who are representing Mr. Christensen, we all appreciate the depth of pain that Ms. Zhang's family is feeling and how difficult this all is for them," Bruno said.

Christensen's trial is set for Sept. 12.