MIAMI  —  Alleged Al Qaeda operative Jose Padilla suffers from intense stress and anxiety after being imprisoned in isolation for years and cannot adequately help his lawyers prepare for a criminal trial, a mental expert testified Thursday.

Dr. Angela Hegarty, a forensic neuropsychiatrist, said she concluded after examining and testing Padilla for more than 22 hours last year that he suffers from post-traumatic stress disorder and is mentally incompetent for trial.

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Padilla's symptoms are most acute when he is asked about his 3 1/2 years in custody at a Navy brig or to review evidence in the case such as transcripts of telephone conversations, she said.

"He doesn't want to because it hurts so much, and because it hurts so much he shuts down," Hegarty said.

She was the first witness at a hearing on Padilla's competency, which is crucial in deciding whether he and two co-defendants will stand trial in April.

Padilla, a 36-year-old U.S. citizen, is charged with being part of a North American terror support cell that provided money, recruits and supplies to Islamic extremists around the world. All three have pleaded not guilty and face possible life imprisonment.

The Bush administration initially claimed that Padilla was on an Al Qaeda mission to detonate a radioactive "dirty" bomb in a major U.S. city when he was arrested in May 2002 at Chicago's O'Hare International Airport.

He was designated an "enemy combatant" and was imprisoned by the military without criminal charges. But the "dirty bomb" allegations are not part of the Miami case.

During cross-examination, prosecutor John Shipley pointed to a test administered by Hegarty in which Padilla scored "zero" on the portions indicating post-traumatic stress disorder. Those segments involved questions about flashbacks, nightmares, depression and other symptoms.

"Nothing in this test supports your diagnosis at all, isn't that correct?" Shipley asked.

"No," Hegarty replied, noting that the test answers were only one component of her decision.