WASHINGTON  Red Cross investigators concluded last year in a secret report that the Central Intelligence Agency’s interrogation methods for high-level Qaeda prisoners constituted torture and could make the Bush administration officials who approved them guilty of war crimes, according to a new book on counterterrorism efforts since 2001.

The book says that the International Committee of the Red Cross declared in the report, given to the C.I.A. last year, that the methods used on Abu Zubaydah, the first major Qaeda figure the United States captured, were “categorically” torture, which is illegal under both American and international law.

The book says Abu Zubaydah was confined in a box “so small he said he had to double up his limbs in the fetal position” and was one of several prisoners to be “slammed against the walls,” according to the Red Cross report. The C.I.A. has admitted that Abu Zubaydah and two other prisoners were waterboarded, a practice in which water is poured on the nose and mouth to create the sensation of suffocation and drowning.

The book, “The Dark Side: The Inside Story of How the War on Terror Turned Into a War on American Ideals,” by Jane Mayer, who writes about counterterrorism for The New Yorker, offers new details of the agency’s secret detention program, as well as the bitter debates in the administration over interrogation methods and other tactics in the campaign against Al Qaeda.