A Guantanamo prisoner often described as the '20 hijacker' in the September 11 attacks was tortured by his American interrogators, a senior official at the Pentagon has admitted.

Mohammed Al-Qahtani's interrogations at Guantanamo in 2002 and 2003, which included sleep deprivation and exposure to cold had been described by officials as abusive.

But the Pentagon has always refused to acknowledge that the treatment of the Saudi national amounted to torture.

However Susan J Crawford, the senior official at the Pentagon responsible for prosecuting detainees has told The Washington Post that she decided last May not to refer his case for trial because she had concluded that he had been tortured.

“His treatment met the legal definition of torture. And that’s why I did not refer the case” Ms. Crawford, a retired military judge, told Bob Woodward of the Washington Post.

She said she came to the conclusion after studying the combination of techniques used on him which she said had a 'medical impact.'

"The techniques they used were all authorized, but the manner in which they applied them was overly aggressive and too persistent," she said.

"You think of torture, you think of some horrendous physical act done to an individual. This was not any one particular act; this was just a combination of things that had a medical impact on him, that hurt his health. It was abusive and uncalled for. And coercive. Clearly coercive. It was that medical impact that pushed me over the edge" to call it torture, she added.

Military documents show that Mr. al-Qahtani’s repeated interrogations included prolonged isolation, sleep deprivation, forced nudity and exposure to cold. He was forced to dance with a male interrogator and to act like a dog, obeying such commands as “stay,” “come” and “bark.”

A Pentagon inquiry in 2005 found that the methods were “degrading and abusive.” Mr. Qahtani’s lawyers at the Center for Constitutional Rights in New York said they left him a broken man who has attempted suicide.

He had been denied entry to the US in August 2001, a month before the attacks on the Twin Towers. He was later captured in Afghanistan and taken to Guantanamo in 2002 where he was accused of plotting the attacks, alongside five other Guantanamo detainees.

Military prosecutors sought the death penalty but in May, Ms Crawford decided not to refer his case for trial. At the time she refused to offer an explanation.

Today she defended his continued detention, describing him as a "muscle hijacker".

“There’s no doubt in my mind he would’ve been on one of those planes had he gained access to the country,” Ms. Crawford said in the interview. "What do you do with him now if you don’t charge him and try him? I would be hesitant to say, ‘Let him go,’ ” she added

Ms Crawford,who served as general counsel for the Army during the Reagan administration, and was the Pentagon's inspector general when Dick Cheney was secretary of defense, is the first senior Bush administration official responsible for reviewing practices at Guantanamo to publicly state that a detainee was tortured.

In a statement on Tuesday, the Pentagon said that more than a dozen investigations into Mr al-Qahtani's treatment had concluded that the interrogations were lawful.

“However, subsequent to those reviews,” the statement said, “the department adopted new and more restrictive policies and improved oversight procedures for interrogation and detention operations.

“Some of the aggressive questioning techniques used on al-Qahtani, although permissible at the time, are no longer allowed in the updated Army field manual,” the statement said.

In November, military prosecutors indicated they would file new charges with Ms Crawfor,.based on subsequent interrogations that did not employ harsh techniques.

But Ms Crawford, who dismissed war crimes charges against him in May 2008, told MR Woodward she would not allow the prosecution to go forward.

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