“Competent staff work could have quickly canvassed relevant history, insights from the best law enforcement and military interrogators, and lessons from the painful British and Israeli experience,” Mr. Zelikow said. “Especially in a time of great stress, walking into this minefield, the president was entitled to get the most thoughtful and searching analysis our government could muster.”

After years of recriminations about torture and American values, Bush administration officials say it is easy to second-guess the decisions of 2002, when they feared that a new attack from Al Qaeda could come any moment.

If they shunned interrogation methods some thought might work, and an undetected bomb or bioweapon cost thousands of lives, where would the moral compass point today? It is a question that still haunts some officials. Others say that if they had known the full history of the interrogation methods or been able to anticipate how the issue would explode, they would have advised against using them.

This account is based on interviews with more than two dozen current and former senior officials of the C.I.A., White House, Justice Department and Congress. Nearly all, citing the possibility of future investigations, shared their recollections of the internal discussions of a classified program only on condition of anonymity.

Leaked to the news media months after they were first used, the C.I.A.’s interrogation methods would darken the country’s reputation, blur the moral distinction between terrorists and the Americans who hunted them, bring broad condemnation from Western allies and become a ready-made defense for governments accused of torture. The response has only intensified since Justice Department legal memos released last week showed that two prisoners were waterboarded 266 times and that C.I.A. interrogators were ordered to waterboard one of the captives despite their belief that he had no more information to divulge.

But according to many Bush administration officials, including former Vice President Dick Cheney and some intelligence officers who are critics of the coercive methods, the C.I.A. program would also produce an invaluable trove of information on Al Qaeda, including leads on the whereabouts of important operatives and on terror schemes discussed by Al Qaeda. Whether the same information could have been acquired using the traditional, noncoercive methods that the Federal Bureau of Investigation and the military have long used is impossible to say, and former Bush administration officials say they did not have the luxury of time to develop a more patient approach, given that they had intelligence warnings of further attacks.

Michael V. Hayden, who served as C.I.A. director for the last two years of the Bush administration, devoted part of his last press briefing in January to defending the C.I.A. program. “It worked,” Mr. Hayden insisted.