C.I.A. interrogators used waterboarding, the near-drowning technique that top Obama administration officials have described as illegal torture, 266 times on two key prisoners from Al Qaeda, far more than had been previously reported.
The C.I.A. officers used waterboarding at least 83 times in August 2002 against Abu Zubaydah, according to a 2005 Justice Department legal memorandum. Abu Zubaydah has been described as a Qaeda operative.
A former C.I.A. officer, John Kiriakou, told ABC News and other news media organizations in 2007 that Abu Zubaydah had undergone waterboarding for only 35 seconds before agreeing to tell everything he knew.
The 2005 memo also says that the C.I.A. used waterboarding 183 times in March 2003 against Khalid Shaikh Mohammed, the self-described planner of the Sept. 11, 2001, terrorist attacks.
The New York Times reported in 2007 that Mr. Mohammed had been barraged more than 100 times with harsh interrogation methods, causing C.I.A. officers to worry that they might have crossed legal limits and to halt his questioning. But the precise number and the exact nature of the interrogation method was not previously known.
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.
President Obama plans to visit C.I.A. headquarters Monday and make public remarks to employees, as well as meet privately with officials, an agency spokesman said Sunday night. It will be his first visit to the agency, whose use of harsh interrogation methods he often condemned during the presidential campaign and whose secret prisons he ordered closed on the second full day of his presidency.
C.I.A. officials had opposed the release of the interrogation memo, dated May 30, 2005, which was one of four secret legal memos on interrogation that Mr. Obama ordered to be released last Thursday.
Mr. Obama said C.I.A. officers who had used waterboarding and other harsh interrogation methods with the approval of the Justice Department would not be prosecuted. He has repeatedly suggested that he opposes Congressional proposals for a “truth commission” to examine Bush administration counterterrorism programs, including interrogation and warrantless eavesdropping.
The Senate Intelligence Committee has begun a yearlong, closed-door investigation of the C.I.A. interrogation program, in part to assess claims of Bush administration officials that brutal treatment, including slamming prisoners into walls, shackling them in standing positions for days and confining them in small boxes, was necessary to get information.
The fact that waterboarding was repeated so many times may 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 Thursday said waterboarding was used both more frequently and with a greater volume of water than the C.I.A. rules permitted.
The new information on the number of waterboarding episodes came out over the weekend when a number of bloggers, including Marcy Wheeler of the blog emptywheel, discovered it in the May 30, 2005, memo.
The sentences in the memo containing that information appear to have been redacted from some copies but are visible in others. Initial news reports about the memos in The New York Times and other publications did not include the numbers.
Michael V. Hayden, director of the C.I.A. for the last two years of the Bush administration, would not comment when asked on the program “Fox News Sunday” if Mr. Mohammed had been waterboarded 183 times. He said he believed that that information was still classified.
A C.I.A. spokesman, reached Sunday night, also would not comment on the new information.
Mr. Hayden said he had opposed the release of the memos, even though President Obama has said the techniques will never be used again, because they would tell Al Qaeda “the outer limits that any American would ever go in terms of interrogating an Al Qaeda terrorist.”
He also disputed an article in The New York Times on Saturday that said Abu Zubaydah had revealed nothing new after being waterboarded, saying that he believed that after unspecified “techniques” were used, Abu Zubaydah revealed information that led to the capture of another terrorist suspect, Ramzi Binalshibh.
The Times article, based on information from former intelligence officers who spoke on condition of anonymity, said Abu Zubaydah had revealed a great deal of information before harsh methods were used and after his captors stripped him of clothes, kept him in a cold cell and kept him awake at night. The article said interrogators at the secret prison in Thailand believed he had given up all the information he had, but officials at headquarters ordered them to use waterboarding.
He revealed no new information after being waterboarded, the article said, a conclusion that appears to be supported by a footnote to a 2005 Justice Department memo saying the use of the harshest methods appeared to have been “unnecessary” in his case.