The United States Federal Bureau of Investigation believes that at least two - and possibly four - of the hijackers implicated in the attacks on the World Trade Center and the Pentagon entered the country on student visas.
Washington lawmakers have been horrified to learn how little information the Immigration and Naturalisation Service holds on foreign students, despite the warning signal of the 1993 bomb attack on the WTC, when one of the convicted attackers was found to hold an expired student visa.
President Bush's directive to tighten controls on international student visas has been welcomed by the academic community and immigration officials. But their superficial unanimity barely conceals deep-seated antagonism.
Since 1988, the INS has required colleges to supply student data on request. After 1993's attack, the Student and Exchange Visitor Information Service (Sevis) electronic database was established to track international students. But the system is being piloted at only 20 campuses, and its development has been delayed by a row between the colleges and the INS.
Michael Becraft, INS acting deputy commissioner, told Congress that officials would not readily know if anyone arriving on a student visa attended classes.
His boss, INS commissioner James W. Ziglar, told a Senate committee: "The INS maintains limited records on foreign students and is able to access that information on demand. But the information is on old technology platforms that are insufficient for today's need for rapid access."
Mr Ziglar said Sevis was the solution but "objections, primarily from the academic establishment, have delayed its deployment". September 11 had "virtually" removed those objections, he said, and the INS would meet Congress's January 2003 implementation deadline.
David Ward, president of the American Council on Education, told a congressional committee: "Ace and most other higher education associations have never opposed the idea behind Sevis, but we have repeatedly expressed our conviction that Sevis should be designed in a way that it does not become a barrier to the enrolment of international students. Regrettably, the INS has never been sensitive to these concerns."
Dr Ward, whose association represents 1,800 public and private colleges and universities, agreed that a review of the policies and procedures for tracking student visas was urgently needed. "We favour having as many international students enrolled at American colleges as possible. However, we do not want to enrol any student that the federal government believes poses a security risk."