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.”