Brendt Christensen, charged with kidnapping Chinese scholar Zhang Yingying, pleaded not guilty during his arraignment at a federal court in Urbana, the US state of Illinois on Thursday.

It was the third time the 28-year-old suspect appeared at the court, and the first time, he broke his silence and spoke for some minutes.

While asked about his mental health conditions by US Magistrate Judge Eric Long, Christensen said he was neither under a doctor's care nor had been in a hospital, but he said he is taking antidepressants.

Some of Zhang Yingying's family members, who have been staying in the US for the case also attended the arraignment.

Christensen's attorney Anthony Bruno said in a statement that he has demanded a jury trial and plans to persist in his plea of not guilty.

He said the case could go on for a year or more.

A pretrial hearing will be held on August 28, and a trial has been scheduled for September 12. If convicted of kidnapping, Christensen might face the maximum penalty of life in prison.

Zhang Yingying, a  26-year-old visiting scholar to the University of Illinois at Urbana-Champaign (UIUC), is missing since June 9, after seen entering a black Saturn Astra car. The police tracked down Christensen and arrested him on June 30. 

The FBI officials said Christensen's words are inconsistent.

At first, he said he could not remember where he was at the time Zhang disappeared, but later he admitted to having picked up an Asian woman at a corner, but let her get off after a wrong turn. 

Investigators later found that Christensen visited a forum named "Abduction 101," and he was also seen attending a vigil held for Zhang.

The 28-year-old was indicted on July 12 by a federal grand jury related to Zhang's abduction and has been held in custody since then. Zhang's whereabouts remain unknown, and the FBI officials believed that she is dead.

Professor Ronald J. Allen from the Northwestern University School of law says that a "plea agreement" may help locate Zhang Yingying.

"When the defendant pleads guilty and gets a certain sentence, that can be negotiated. This is a controversial practice, but it exists in the United States, in the system. Everywhere functions in this way. As soon as an indictment comes down, government will go to his lawyer and say, look, tell us where the remains are and maybe we can reduce that a little bit. That is what I think is going on right now,” Professor Allen said.

A plea agreement is made between the prosecutor and the defendant in a criminal case. The defendant agrees to plead guilty to a particular charge in return for some concessions from the prosecutor. Both parties thereby avoid a lengthy criminal trial.. 

Over 90 percent of criminal cases in the United States are processed through such deals. They also allow defendants to avoid the risk of conviction on a more serious charge. But the absence of a judge and jury may result in injustices or even a false conviction.