The torture technique of waterboarding carried out by the U.S. Central Intelligence Agency against high-level suspects of the September 11 attacks may be far more brutal than previous details have described.

According to reporting by the UK's Telegraph on Sunday, which cites an anonymous security source "who has first-hand knowledge of the period," alleged Sept. 11 attack mastermind Khalid Sheikh Mohammed and at least one other person suffered "medieval" treatment by the CIA and were waterboarded "until the point of death."

According to a description of waterboarding given by intelligence officers to ABC News in 2005:

The prisoner is bound to an inclined board, feet raised and head slightly below the feet. Cellophane is wrapped over the prisoner's face and water is poured over him. Unavoidably, the gag reflex kicks in and a terrifying fear of drowning leads to almost instant pleas to bring the treatment to a halt.

The CIA Inspector General report in 2004, however, found that the waterboarding as carried out by CIA operatives was used in a harsher way than was permitted by the infamous 2002 Bybee memo, in part by its greater frequency of use and of the interrogator dumping "large volumes of water to a cloth that covered the detainee’s mouth and nose."

Waterboarding has been denounced by United Nations officials as torture.

But the new reporting by the Telegraph shows that the actual waterboarding carried out by the CIA may have been far more gruesome, involving not just pouring water onto detainees to stimulate drowning but submerging the men fully in water.

From the 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.” [...] “They got medieval on his ass, and far more so than people realise,” the source told The Telegraph referring to the treatment of Mohammed and [Abd al-Rahim] Nashiri, but declined to provide further details because of the still-classified nature of the material.

Then-director of CIA Michael Hayden admitted in 2008 to Congress that the CIA had used waterboarding against three high-level al Qaeda suspects — Khalid Sheikh Mohammed, Abu Zubaydah and Abd al-Rahim al-Nashiri — and a 2005 Justice Department legal memo revealed that Mohammed was waterboarded 183 times in March 2003.

In a memoir, former President George W. Bush recounted that when he was asked by the CIA if the agency could use waterboarding against Mohammed, his reply was, "Darn right." Senior Bush administration officials including Condoleezza Rice reportedly gave the OK to the torture technique as early as 2002.

How the waterboarding was exacted against the three men, in any of its torturous forms, remains unclear, as in 2007 it was revealed that the CIA had destroyed video tape evidence of waterboarding of Abu Zubayda. Of his 2005 order to destroy the tapes, former head of the CIA's Counterterrorism Center, Jose Rodriguez, said he was "just getting rid of some ugly visuals."

The Telegraph's news of the waterboarding allegations follows the Washington Post's reporting last month that the militant group ISIS waterboarded four hostages it was holding in Syria, an apparent copying of the same technique the CIA used against suspected terrorists involved in the 9/11 attacks. From the Post: “'They knew exactly how it was done'” a person with direct knowledge of what happened to the hostages said of the Islamic State militants."

The summary of the U.S. Senate Intelligence Committee's report on post-9/11 CIA interrogation techniques may be available to the public by the end of this month.