WASHINGTON  Even the most exacting truth commission may have a hard time determining for certain whether brutal interrogations conducted by the Central Intelligence Agency helped keep the country safe.

Last week’s release of long-secret Justice Department interrogation memorandums has given rise to starkly opposing narratives about what, if anything, was gained by the C.I.A.’s use of waterboarding, wall-slamming and other physical pressure to shock and intimidate Qaeda operatives.

Senior Bush administration officials, led by Vice President Dick Cheney and cheered by many Congressional Republicans, are fighting a rear-guard action in defense of their record. Only by using the harshest methods, they insist, did the intelligence agency get the information it needed to round up Qaeda killers and save thousands of American lives.

Even President Obama’s new director of national intelligence, Dennis C. Blair, wrote in a memorandum to his staff last week that “high value information came from interrogations in which these methods were used,” an assertion left out when the memorandum was edited for public release. By contrast, Mr. Obama and most of his top aides have argued that the use of those methods betrayed American values  and anyway, produced unreliable information. Those are a convenient pair of opinions, of course: the moral balancing would be far trickier if the C.I.A. methods were demonstrated to have been crucial in disrupting major plots.