ASHINGTON, May 17 ó The F.B.I. had been aware for several years that Osama bin Laden and his terrorist network were training pilots in the United States and elsewhere around the world, according to court records and interviews at flight schools and with federal law enforcement officials.
The F.B.I. knew by 1996 of a specific threat that terrorists in Al Qaeda, Mr. bin Laden's network, might use a plane in a suicide attack against the headquarters of the C.I.A. or another large federal building in the Washington area, the law enforcement officials acknowledged.
But the officials said the Federal Bureau of Investigation had discounted the possibility of a suicide attack using planes, partly because it had largely failed to draw together evidence gathered piecemeal over the years that Al Qaeda pilots were training here.
Last week, the F.B.I. acknowledged the existence of a memorandum written last summer in which an agent in its Phoenix office urged his superiors to investigate Middle Eastern men who had enrolled at American flight schools and who might be connected to Mr. bin Laden.
However, the Phoenix memorandum was not the first warning that terrorists affiliated with Al Qaeda had interest in learning to fly. In his 1996 confession, a Pakistani terrorist, Abdul Hakim Murad, said that he planned to use the training he received at flight schools in the United States to fly a plane into C.I.A. headquarters in Langley, Va., or another federal building.
Mr. Murad, who was captured in the Philippines in 1995 and convicted in New York on charges of conspiring to blow up 12 American jumbo jets over the Pacific simultaneously, received flight training at schools in New York, North Carolina, California and Texas.
Information from Mr. Murad's confession formed a basis for an analysis prepared for United States intelligence agencies in 1999. The analysis warned that bin Laden terrorists could hijack a jet and fly it into government buildings like the Pentagon.
Additionally, a flight school in Oklahoma that provided training last year to Zacarias Moussaoui, the so-called 20th hijacker in the Sept. 11 attacks, had been under scrutiny by the F.B.I. in 1998 after the discovery that another former student had been linked to the bombing that year of two American embassies in East Africa, attacks attributed to Al Qaeda. The school, the Airman Flight School in Norman, has cooperated with the F.B.I.
Congressional investigators say they are only now compiling a detailed chronology of what was known about potential terrorists receiving flight training here as Congress evaluates whether the F.B.I. and other law enforcement agencies failed to recognize signs that might have allowed the government to prevent the September attacks. At least six of the Sept. 11 hijackers received flight training in the United States.
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 ó 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.
"I don't think anybody could have predicted that these people would take an airplane and slam it into the World Trade Center," Condoleezza Rice, the national security adviser, said on Thursday.
But since at least the mid-1990's, law enforcement officials have known that some terrorist organizations were considering suicide attacks using commercial jets.
In 1994, French investigators have said, a group of Algerian hijackers seized a Paris-bound Air France flight and planned to crash it into the Eiffel Tower or blow it up over Paris. The plot was foiled when French commandoes stormed the plane.
In 1995, Mr. Murad, the Pakistani pilot tied to Mr. bin Laden, was captured, and under interrogation by Philippines intelligence officers working with the F.B.I. and the C.I.A., American law enforcement officials said, he confessed on video to his role in the plot to bomb airliners over the Pacific.
The American officials said he also acknowledged he had planned to fly a plane packed with explosives into the C.I.A. headquarters or another federal building. Details of the plan had been shared with F.B.I. headquarters by the middle of 1996.