Zacarias Moussaoui is a paranoid schizophrenic, a defence witness has told the jury in his sentencing trial in the US state of Virginia.
Moussaoui has said he has no remorse over the attacks
Psychologist Xavier Amador also said Moussaoui believed President George W Bush would release him.
The jury has to decide whether Moussaoui should be executed for his role in the 11 September 2001 attacks.
Defence lawyers are trying to persuade jurors to spare his life because there is evidence he is mentally ill.
Moussaoui, a French citizen of Moroccan descent, is the only person convicted in the US in connection with the attacks on New York and Washington.
Moussaoui was in jail in Minnesota at the time, but in an earlier phase of the trial prosecutors successfully argued that he had kept federal agents from identifying and stopping some of the hijackers.
'Beautiful terrorist mind'
Mr Amador said Moussaoui was convinced his defence lawyers were out to kill him and believed the FBI was bugging him before 11 September.
Mr Amador said Moussaoui's behaviour was not due to his fundamentalist beliefs, but to his paranoid schizophrenia.
At one point, Moussaoui shouted out "beautiful terrorist mind", apparently in reference to remarks made by Mr Amador on Monday, about the case of mathematician John Nash, a schizophrenic whose life story was made into the film, A Beautiful Mind.
Mr Amador said that in an hour-long meeting with the defendant in his cell, Moussaoui had been agitated and aggressive.
He told the court that when he arrived, he found wet paper towels strewn across the floor of the cell and learned that Moussaoui had been spitting water at his guards. Moussaoui then proceeded to drench him as well, and refused to speak to him, Mr Amador said.
Eventually Moussaoui relented, at which point he became depressed and lost his "adversarial" demeanour.
"He said softly, 'Bush will release me'," Mr Amador said.
Mr Amador stressed to the court that Moussaoui "really believes that", at which point the defendant nodded emphatically.
At other times, Moussaoui frowned and shook his head as the psychologist described what he called evidence of delusional states.
On Monday, the court heard that Moussaoui came from a broken home where his mother was repeatedly beaten, and that there was a history of mental illness in his family.
The prosecution wrapped up its case last week, after presenting the jury with emotional evidence from the 11 September attacks.