The CIA brought top al-Qaeda suspects close to the point of death" by drowning them in water-filled baths during interrogation sessions in the years that followed the September 11 attacks, a security source has told The Telegraph. The description of the torture meted out to at least two leading al-Qaeda suspects, including the alleged 9/11 mastermind Khalid Sheikh Mohammed, far exceeds the conventional understanding of waterboarding, or "simulated drowning" so far admitted by the CIA. "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 account of extreme CIA interrogation comes as the US Senate prepares to publish a declassified version of its so-called Torture Report - a 3,600-page report document based on a review of several million classified CIA documents. Publication of the report is currently being held up by a dispute over how much of the 480-page public summary should remain classified, but it is expected to be published within weeks. A second source who is familiar with the Senate report told The Telegraph that it contained several unflinching accounts of some CIA interrogations which - the source predicted - would "deeply shock" the general public.