CIA interrogators considered using so-called truth serum on captured terror suspects after the 9/11 attacks, according to a de-classified report.

The agency spent months researching drugs - none of which have shown any clear scientific evidence of effectiveness - as an "benign alternative" to waterboarding in the hope of forcing detainees to give up details of Al Qaeda plots.

It eventually chose the sedative Versed only to decide not to go ahead with the program in early 2003.

It discloses that the use of truth drugs was raised in August 2002 following lengthy interrogation of Abu Zubaydah, who was thought to be a senior member of Al Qaeda with knowledge of existing plots and 9/11.

The detainee was repeatedly subjected to “waterboarding“, a technique which has been described as torture and involves pouring water on to a cloth covering the mouth and nose to invoke the sensation of of drowning.

He proved “amazingly resistant” to the technique, according to the report, and insisted he had given up everything he knew.

“The intensity and duration of AZ’s interrogation came as a surprise to OMS [the Office of Medical Services] and prompted further study of the.seemingly more benign alternative of drug-based interviews,” the report reads.

“Such drugs, although widely regarded as unreliable sources of truth, were believed potentially useful an an excuse that would allow the subject to be more forthcoming while still saving face.”

Researchers studied records of old Soviet drug and “mind control” experiments as well the CIA's experiments with LSD on humans during the 1950s and 1960s.

Psychiatrists were consulted about the use of substances such as sodium amytal before staff settled on Versed, a brand name for the sedative midazolam.

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The drug causes drowsiness, relieves anxiety and agitation and can temporarily impair memory. It is often is used for minor surgery or medical procedures such as colonoscopies that require sedation but not full-blown anaesthesia.

“Versed was considered possibly worth a trial if unequivocal legal sanction first were obtained,” the report said. “There were at least two legal obstacles: a prohibition against medical experimentation on prisoners and a ban on interrogational use of ‘mind-altering drugs’ or those which ‘profoundly altered the senses.”’

The CIA eventually decided not to ask the Justice Department for permission and the project came to an end.

Its decision not to use “truth” drugs spared doctors “some significant ethical concerns”, according to the author of the report.

“Whether or not consent was obtained drug administration – presumably by a·physician – clearly would have been an invasive procedure for non-therapeutic reasons," it said.

However the ACLU claim that the document reveals how medical staff were “critical participants in experimenting with torture”.

“Just like the government lawyers who tried to give unlawful torture a veneer of legality, the secret history reveals that CIA doctors were indispensable to the effort of legitimising the program,” said ACLU attorney Dror Ladin.

“Perhaps the most striking element of the document is the CIA doctors’ willful blindness to the truth of what they were doing.”

The voices of 9/11: The ripples still being felt around the world Show all 4 1 /4 The voices of 9/11: The ripples still being felt around the world The voices of 9/11: The ripples still being felt around the world The voices of 9/11: The ripples still being felt around the world The voices of 9/11: The ripples still being felt around the world What remains: A New York City firefighter looks up at what remains of the World Trade Centre on 13 September, 2001 REUTERS The voices of 9/11: The ripples still being felt around the world Five days on: FEMA workers survey the wreckage of the World Trade Centre on September 16, 2001 REUTERS

Medical staff believed waterboarding was not “intrinsically painful” and in one case “provided periodic relief” to a prisoner after days of sleep deprivation.

Confinement in a coffin-sized box was also described as offering a “relatively benign sanctuary” from interrogation

While the CIA interrogation program came to an end in 2007, Donald Trump declared during his presidential election campaign that he would bring back waterboarding.