The CIA tortured them by drowning suspects, including alleged 9/11 mastermind Khalid Sheikh Mohammed, in baths when conducting interrogation, a security source told The Daily Telegraph.

"They weren't just pouring water over their heads or over a cloth," said the source, who has first-hand knowledge of the period.

"They were holding them under water until the point of death, with a doctor present to make sure they did not go too far. This was real torture," the source added.

The US Senate is preparing to publish a declassified version of its so-called "Torture Report," which is a 3,600-page document after reviewing several million classified CIA documents.

However, publication of the report is being held up over how much of the public summary should be kept classified. The report is expected to be published within weeks.

A second source familiar with the Senate report said it has several unflinching accounts of some CIA interrogations which, the source predicted, would "deeply shock" the public.

Dianne Feinstein, who chairs the Senate Intelligence Committee, has said the report will reveal “brutality that stands in stark contrast to our values as a nation.”

The torture technique is said to be much worse than the waterboarding or simulated drowning. The CIA has already been under fire from rights groups for the controversial technique which amounts to torture. 

RA/NJF