The FBI agent who wrote the so-called Phoenix memo has told US legislators he recommended that hundreds or even thousands of Middle Eastern students entering the country be screened by officials.
But Kenneth Williams said that even if such action had been taken, it would not have prevented the attacks of 11 September.
Mr Williams, 41, a former Swat team member, wrote an internal FBI memo on 10 July in which he expressed his concerns about a number of flight students he was monitoring in Phoenix, Arizona. While he did not specify that those students had links to al-Qa'ida, he raised the prospect that the terror network could use American flight schools to train its members to launch attacks on US targets. Most of the 11 September hijackers spent some time at American flight schools.
Mr Williams, previously a low-profile, middle-ranking officer, gave testimony on Tuesday evening before a closed-door session of the Senate Judiciary committee. He gave similar information yesterday to a closed meeting of a joint House-Senate intelligence committee investigating the events leading up to 11 September.
Senator Richard Durbin of Illinois, who attended the hearing on Tuesday, said: "The catalyst for his memo was the very specific statements by the students that he had interviewed at the flight schools. What they say about the United States was chilling." He added: "Every indication was that the traffic light went from yellow to red and the FBI just kept driving.
"If Washington had taken that memo as seriously as the agent did, I think we would have been on a much greater state of alert before September 11."
Mr Williams and his now notorious memo are at the centre of an increasingly heated and politically partisan debate on what the Bush administration knew before 11 September about possible attacks. In the days immediately after the attacks on New York and Washington, various administration figures insisted there were no warnings – something that now seems not to be the case.
Mr Bush, who was warned in an intelligence briefing on 6 August of the threat of al-Qa'ida hijacking aircraft, had to agree to release to Congress some of the intelligence documents relating to possible attacks.
Such limited disclosures are unlikely to placate those Democrats seeking a special commission to investigate the matter. The Senate majority leader, Tom Daschle, is the latest high-ranking Democrat to call for such an independent inquiry. "The time has come for us to do what they did after the invasion of Pearl Harbor ... to ensure that we get all the facts," he said.
Yesterday The Washington Post reported that Mr Williams' memo was formally rejected within weeks after mid-level officials decided they did not have the necessary manpower.