Susan Crawford, the Bush administration official in charge of deciding whether to bring Guantanamo detainees to trial, told The Washington Post that the suspect cannot be tried because of the torture.
Mohammed al-Qahtani, 30, is alleged to have been the so-called 20th hijacker in the 11 September 2001 attacks on the US.
US military interrogators subjected him to sustained isolation, sleep deprivation, nudity and prolonged exposure to cold, leaving him in what Ms Crawford said was a life-threatening condition.
She told the Post: 'His treatment met the legal definition of torture. And that's why I did not refer the case (for prosecution).'
Ms Crawford, a retired judge who previously worked for the Pentagon, dismissed war crimes charges against Mr Qahtani in May 2008.
