The official overseeing trials at Guantanamo Bay says US agents tortured a Saudi man suspected of involvement in the 11 September attacks.
Susan Crawford told the Washington Post newspaper that Mohammad al-Qahtani had been left in a "life-threatening condition" after being interrogated.
The Pentagon said their methods were legal in 2002, when the interviews took place - though some were now banned.
The BBC reports Mr Qahtani remains at Guantanamo, but all charges against him were dropped.
He was facing trial on counts of conspiracy, terrorism, and murder in violation of the laws of war.
Ms Crawford said he had been subjected to sustained periods of cold, isolation and sleep deprivation and was interrogated for 18 to 20 hours a day almost continuously for eight weeks.
The BBC reports Mr Qahtani has been in detention at Guantanamo since 2002, after being picked up in Afghanistan.
The US authorities accused him of intending to take part in the 11 September attacks, and he was labelled the "20th hijacker".
Despite her decision to drop the prosecution, Ms Crawford said Mr Qahtani remained a "very dangerous man".