The Phoenix memorandum, along with the disclosure this week that President Bush was warned in August of the possibility that Al Qaeda might be planning hijackings, have been seized on by lawmakers as evidence that the government missed signals of the coming attacks.

Spokesmen for F.B.I. headquarters in Washington, as well as for its field offices in New York and Oklahoma City, which investigated individual flight schools over the years, had no comment on the issue.

Lewis Schiliro, who retired two years ago as the bureau's assistant director in charge of its New York office, said in an interview that his agents had tried to follow up on information about the flight schools whenever possible. But he said that while the F.B.I. worried that Al Qaeda might hijack commercial planes, ''never once did we really focus on the use of a plane as a weapon'' and that it would have been ''very difficult to connect the dots.''

The F.B.I. did not alert other federal agencies about many of the results of its flight school investigations. The Phoenix memorandum, in fact, was sent to the Central Intelligence Agency only in recent weeks. The bureau's failure to alert other agencies is expected to be a focus of Congressional investigations into intelligence failures before Sept. 11.

Senator Charles E. Grassley of Iowa, a prominent critic of the F.B.I. and a senior Republican on the Judiciary Committee, said in a statement today that ''it's clear the intelligence community had information about terrorist threats and hijacking years before the F.B.I. agent in Phoenix sent his warning memo -- that makes it even more indefensible that the F.B.I. failed to deal with the Phoenix memo last summer.''

Law enforcement officials acknowledged that the F.B.I. never ordered a comprehensive investigation of flight schools before Sept. 11, even as individual F.B.I. offices were gathering compelling evidence about links between students trained at the schools and Al Qaeda.

In response to the uproar after the disclosure of the August warning to Mr. Bush, White House officials insisted that they had no serious evidence last summer that Al Qaeda was considering a suicide hijacking.