Al-Qaeda plotter Zacarias Moussaoui's social isolation made him vulnerable to recruitment by militants, the jury in his sentencing trial has been told.
Moussaoui has said he has no remorse over the attacks
Psychologist Paul Martin, a specialist in cults, told a US court that many French-Moroccans like Moussaoui felt alienated from Western society.
The trial is due to decide whether Moussaoui should be executed for his role in the 11 September 2001 attacks.
The defence wants jurors to spare his life because he is mentally ill.
On Monday and Tuesday, defence witnesses testified that Moussaoui was a paranoid schizophrenic who came from a family with a history of mental illness.
Moussaoui, who is at odds with his own defence team, insists he is not crazy.
He 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.
Dr Martin told the court in Virginia that Moussaoui was spiritually adrift in the early 1990s, when he began his move towards radical Islam.
"He's away from his family. He's lonely. He's complained about racism. He's in a new country, and he doesn't have any support group," Dr Martin said.
By the time Moussaoui arrived in the UK in 1992, "his plate is full in terms of psycho-social and social problems," said Dr Martin, referring to a lack of housing, money and family support.
At a mosque in the Brixton area of London, he came under the influence of radicals who "said 'look we've got the pure form of Islam, we are moving, we are acting'," Dr Martin added.
However, under cross-examination, Dr Martin said he had neither interviewed Moussaoui nor met him, and had no direct knowledge of the path he took from impressionable Muslim to al-Qaeda operative.
Moussaoui, prone to outbursts during the proceedings, shouted "Moussaoui fly over the cuckoo's nest", as he was led from court for a break.
The prosecution wrapped up its case last week, after presenting the jury with emotional evidence from the 11 September attacks.