CIA agents used waterboarding, the near-drowning interrogation technique, 266 times on two key prisoners from al-Qaeda, it has emerged.
They used waterboarding at least 83 times in August 2002 against Abu Zubaydah, according to a 2005 US Justice Department legal memorandum. Zubaydah has been described as senior al-Qaeda recruiter and trainer. The 2005 memo also says that the CIA used
waterboarding 183 times in March 2003 against Khalid Shaikh Mohammed, the self-described planner of the 11 September, 2001 attacks.
The Senate intelligence committee is investigating the CIA interrogation programme, which under former US president George Bush also included slamming prisoners into walls, shackling them in uncomfortable positions and depriving them of sleep.
The release of the numbers is likely to become part of the debate about the morality and efficacy of interrogation methods that the Justice Department under the Bush administration declared legal, even though the United States had historically treated them as torture. CIA officials had opposed the release of the interrogation memo, dated 30 May, 2005, which was one of four secret legal memos on interrogation that US president Barack Obama ordered to be released last Thursday.
Mr Obama said CIA officers who had used waterboarding and other harsh interrogation methods with the approval of the Justice Department would not be prosecuted.
The president has repeatedly suggested that he opposes congressional proposals for a "truth commission" to examine Bush administration counterterrorism programmes.
And last night, in his first visit to CIA headquarters since ordering the release of the memos, Mr Obama told staff that they must work scrupulously as they stand as a security barrier for Americans who face attack from people who have no scruples.
The president said he understood intelligence officials must sometimes feel that they were working with one hand tied behind their backs, but added that upholding American values in the face of such enemies was "what makes the United States special and what makes you special."
The decision not to seek charges against the interrogators has been criticised by the American Civil Liberties Union and called a violation of international law by the United Nations' top torture investigator.
The new information on the number of waterboarding episodes came out over the weekend when a number of sources discovered it in the 30 May, 2005, memo. The sentences in the memo containing that information appear to have been censored from some copies but are visible in others.
The fact that waterboarding was repeated so many times might raise questions about its effectiveness, as well as about assertions by Bush administration officials that their methods were used under strict guidelines.
A footnote to another 2005 Justice Department memo released on Thursday said waterboarding was used both more frequently and with a greater volume of water than the CIA rules permitted.
Michael Hayden, who led the CIA under Mr Bush, said the release of the memos would make it harder to get useful information from suspected terrorists being detained by the US.
"I think that teaching our enemies our outer limits, by taking techniques off the table, we have made it more difficult in a whole host of circumstances," Mr Hayden said.
