The first witness, Harry Samit, an F.B.I. agent in Minnesota who questioned Mr. Moussaoui at his arrest, firmly asserted that had he been given the truth "we would have several new leads to investigate," and the plot might have been thwarted. Instead, he said, Mr. Moussaoui's answers sent investigators on "wild goose chases."

Under cross-examination by Edward B. MacMahon Jr., a court-appointed lawyer for Mr. Moussaoui, Mr. Samit acknowledged that after the attacks he had written strongly worded reports saying his superiors had improperly blocked his efforts to investigate Mr. Moussaoui. He added that he was convinced that Mr. Moussaoui was a terrorist involved in an imminent hijacking plot.

That senior bureau officials dragged their feet on investigating Mr. Moussaoui by seeking search warrants from a special intelligence court or a more routine criminal search warrant was not new. But it had never been presented so vividly as a reluctant Mr. Samit was obliged to do under cross-examination.

He offered a devastating comment from a supervisor who said pressing too hard to obtain a warrant for Mr. Moussaoui would hurt his career. Mr. Samit also wrote that his superiors did not act because they were guilty of "criminal negligence" and they were gambling that Mr. Moussaoui had little to offer. The lost wager, Mr. Samit said, was paid in many lives. Mr. Samit was followed to the witness stand by Michael Rolince, a retired F.B.I. counterterrorism supervisor who similarly recited a list of actions that the bureau could have taken if Mr. Moussaoui had told them about Qaeda plans to take over planes with knives and fly into buildings.

But when Mr. MacMahon began reading from a document detailing many suspicions about Mr. Moussaoui's intentions, Mr. Rolince interrupted, "Can I ask what document that's coming from?" Mr. MacMahon obliged, noting that it was an urgent memorandum written by Mr. Samit on Aug. 18, 2001, hoping to attract the attention of headquarters. Mr. Rolince had inadvertently underlined that the agent's suspicions had never risen to his attention.