CIA interrogators considered using a truth serum on an al Qaeda suspect after waterboarding and other "enhanced interrogation techniques" failed to get the answers they wanted, according to newly declassified documents.
Abu Zubaydah was believed to have been involved in the planning of the 9/11 attacks. Desperate to find out what he might know about any future terrorist activity, the CIA waterboarded him 83 times, as well using sleep deprivation and stress positions.
When he refused to crack agency doctors looked back at 1950s experiments with mind-altering drugs like LSD and to Russian testing of alleged truth serums in the 1980s.
They also considered a drug called Versed, a sedative that can cause temporary memory loss.
It was thought that by using this a suspect could be tricked into believing they had confessed and thus, with nothing to lose, would give up even more information.
The documents detailing the involvement of the CIA's Office of Medical Services (OMS) in the questioning of suspects were released after a court battle led by the American Civil Liberties Union.
They reveal that the OMS examined details of the 1950s MKULTRA program in which mind-altering drugs were tested on humans. One man who was secretly given LSD in that programme committed suicide.
But they found an absolute lack of historical evidence that drugs could induce a subject to give up information. "No such magic brew as the popular notion of truth serum exists," said a 1961 intelligence review.
"It seems likely that any individual who can withstand ordinary intensive interrogation can hold out in narcosis," it said.
Medical research on prisoners was subsequently banned. It was this ban, along with the lack of any scientific evidence that the drugs would work that prompted doctors not to use them on Zubaydah.
After having stretched legal limits to get permission to use enhanced interrogation techniques on prisoners, the CIA's legal office "did not want to raise another issue with the Department of Justice," the report said.
Abu Zubaydah is a Saudi-born Palestinian national who is still being held in Guantánamo Bay. He was arrested in Pakistan in 2002 and has been in US custody ever since.
As well as waterboarding he was subjected to beatings, isolation, long-time standing, continuous cramped confinement, and sleep deprivation. He lost an eye while being held by the CIA.
More from US
An internal US intelligence memo said he "showed remarkable resilience" to interrogation and it is disputed as to whether he ever provided significant information.
Earlier this year a report from the Commons intelligence and security committee said that MI6 had "direct awareness of extreme mistreatment and possibly torture" of Zubaydah.