Now, in sworn statements that seem likely to reignite the debate, two former senators who were privy to top secret information on the Saudis’ activities say they believe that the Saudi government might have played a direct role in the terrorist attacks.

“I am convinced that there was a direct line between at least some of the terrorists who carried out the September 11th attacks and the government of Saudi Arabia,” former Senator Bob Graham, Democrat of Florida, said in an affidavit filed as part of a lawsuit brought against the Saudi government and dozens of institutions in the country by families of Sept. 11 victims and others. Mr. Graham led a joint 2002 Congressional inquiry into the attacks.

Joining in the latest round of an old dispute, the Democratic senators who lead the intelligence and armed services committees took issue on Monday with claims from Bush administration officials that the Central Intelligence Agency’s coercive interrogation methods produced information that led to the killing of Osama bin Laden a year ago.

Morris Davis says allowing evidence from torture means the world will never see Guantanamo Bay trials as fair

The former chief US prosecutor at Guantanamo Bay has denounced the military trial of Khalid Sheikh Mohammed, the alleged mastermind of the 9/11 attacks due to appear in court at Guantanamo on Saturday, as intended primarily to prevent the defendants from presenting evidence of torture.

Morris Davis, a former colonel who was chief prosecutor when Mohammed was brought to Guantanamo in 2006, said the military commissions will be badly discredited by the use of testimony obtained from waterboarding and other "enhanced interrogation" techniques used on the accused men.

THE United States has been narrowly saved from lethal terrorist plots in recent years — or so it has seemed. A would-be suicide bomber was intercepted on his way to the Capitol; a scheme to bomb synagogues and shoot Stinger missiles at military aircraft was developed by men in Newburgh, N.Y.; and a fanciful idea to fly explosive-laden model planes into the Pentagon and the Capitol was hatched in Massachusetts.

But all these dramas were facilitated by the F.B.I., whose undercover agents and informers posed as terrorists offering a dummy missile, fake C-4 explosives, a disarmed suicide vest and rudimentary training. Suspects naïvely played their parts until they were arrested.