A board reviewing the status of prisoners at Guantanamo Bay has decided against releasing a Saudi who U.S. authorities believe narrowly avoided becoming one of the hijackers in the Sept. 11, 2001, terrorist attack.

Lawyers for prisoner Mohammed al-Qahtani asked the Periodic Review Board last month to send the prisoner to a rehabilitation center in Saudi Arabia for treatment of severe mental illness. The board, made up of representatives of six government agencies, turned down the request in a statement released Wednesday.

The board cited several reasons for its decision, including the fact that al-Qahtani "almost certainly" had been chosen by senior Al Qaeda members to be the 20th hijacker in the Sept. 11 plot and his "refusal to respond to questions" about his past activities.

Al-Qahtani tried to enter the U.S. before the Sept. 11 attack but was turned away by immigration officers at the airport in Orlando, Florida. The suicide attack went ahead with 19 hijackers, killing nearly 3,000 people in New York, Pennsylvania and Washington. He was captured in Afghanistan and in February 2002 taken to the U.S. base in Cuba, where he was subjected to brutal interrogation that a senior Pentagon legal official later said amounted to torture.

The U.S. charged al-Qahtani before a military tribunal along with five other prisoners with war crimes for the Sept. 11 attack. But the charges against him were withdrawn because of his treatment at Guantanamo. The case against the others has been proceeding slowly at the base but no trial date has been set.

Lawyers for al-Qahtani, 41, told the review board that long before he was taken into U.S. custody he suffered psychiatric disabilities that included schizophrenia, major depression and possibly neurocognitive disorder due to a traumatic brain injury sustained in a car accident when he was 8.

One of his lawyers, Shane Kadidal of the Center for Constitutional Rights in New York, said they would not allow members of the board to question al-Qahtani about his past because of his mental illness and made that clear to the board.

"No competent lawyer would allow his mentally ill and traumatized client to answer questions about past conduct when that client had just a few years ago faced military charges punishable by the death penalty," Kadidal said.

The board said it would review the prisoner's case again in six months.