An internal report by the FBI has catalogued a long list of abuses of prisoners held at the US detention centre in Guantanamo Bay in Cuba.
A civil liberties group is filing a suit on behalf of former detainees
At least 26 agency employees say they witnessed inmates being mistreated and subjected to harsh interrogation.
One reported seeing a man whose head was covered in duct tape, another saw detainees chained from hand to foot in the foetal position for up to 24 hours.
The report was released to a US civil liberties group fighting a law suit.
A federal judge has yet to decide whether to accept the suit, filed by the American Civil Liberties Union (Aclu) against former Defence Secretary Donald Rumsfeld and others on behalf of former inmates who say they were abused.
A Pentagon spokesman said the department had a clear policy: "We treat detainees humanely. The United States operates safe, humane and professional detention operations for enemy combatants who are providing valuable information in the war on terror."
Nonetheless, the BBC's Jonathan Beale says the report appears to clearly contradict such assurances, which have also been made by President George W Bush.
In 2004, the FBI asked 493 employees whether they had witnessed aggressive treatment that was not in line with agency policies.
It says it received 26 witness statements detailing abuse that included:
- Rooms kept so hot that a detainee was rendered almost unconscious, with a pile of hair next to him that he apparently pulled out
- Detainees left for over 24 hours chained to the floor hand and foot in foetal position, with no chair, food or water - most urinated or defecated on themselves
- Sleep deprivation interviews with strobe lights and loud music, apparently lasting up to 4 days
- A detainee brought into an interview room appearing to have broken fingers and facial injuries
- A detainee gagged with duct tape that covered much of his head after reportedly chanting the Koran non-stop
- A detainee draped in an Israeli flag in a room with loud music and strobe lights
Many of the allegations are already in the public domain, the FBI said, while stressing that its own personnel were not involved.
The agency's report added that some officials had told FBI agents that the interrogation techniques had been approved at the defence department by officials who included Mr Rumsfeld.
The report does not amount to a criminal investigation, and all the information has been passed on to the Pentagon's inspector general, the FBI said.
It was made in response to a freedom of information request by the Aclu, which has already obtained a number of documented abuse allegations in this manner.