A broad array of signals -- from foiled plots to FBI field interviews -- suggested for years that al Qaeda-affiliated terrorist groups had considered employing airplanes as missiles and U.S. flight schools as pilot training grounds.

The clues included a 1995 plot to blow up 11 American jetliners over the Pacific Ocean, then crash a light plane into CIA headquarters -- a suicide mission to have been carried out by a Pakistani pilot who had trained at flight schools in North Carolina, Texas and New York.

FBI investigators visited two of the flight schools in 1996 after the plot was uncovered in the Philippines, school operators said. In 1998 and 1999, analysts warned federal officials that terrorists might crash hijacked aircraft into landmarks such as the Pentagon and the World Trade Center. Then, last July, the Italian government closed airspace over Genoa and mounted antiaircraft batteries based on information that Islamic extremists were planning to use an airplane to kill President Bush.

"There's a lot of stuff that was out there," said Stephen Gale, a terrorism specialist at the University of Pennsylvania who presented an analysis warning of airborne attacks to Federal Aviation Administration security officials in 1998. "The question is in what form it was out there, who was presenting and collating the information and what was the context in which the information was presented to the president."

The Bush administration, fending off questions about how it acted on intelligence about terrorism before the Sept. 11 attacks, has asserted that although officials received generalized warnings about hijackings, there was no information indicating terrorists affiliated with Osama bin Laden might use hijacked airplanes as manually guided weapons.

National security adviser Condoleezza Rice said Thursday that an intelligence briefing given to Bush on Aug. 6 mentioned hijacking only "in the traditional sense."

"I don't think anybody could have predicted that these people would take an airplane and slam it into the World Trade Center, take another one and slam it into the Pentagon, that they would try to use an airplane as a missile," she said.

White House spokesman Ari Fleischer described Sept. 11 as "a new type of attack that had not been foreseen."

Michael E. O'Hanlon, a national security specialist at the Brookings Institution, said he disagreed with the administration's analysis. The 1995 jetliner bombing plot, he said, signaled that "the traditional hijacking M.O. was not going to be repeated by al Qaeda."

The plot was uncovered when a Pakistani national, Abdul Hakim Murad, was discovered mixing a bomb in his Manila apartment. He later confessed to Philippine authorities that he was part of a conspiracy to deploy five-man teams to plant bombs on 11 planes operated by United, Delta and Northwest airlines. The plot had included a dry run in which a small bomb was exploded under a seat on a Philippine Airlines flight to Tokyo, killing a Japanese businessman. It was orchestrated by Ramzi Yousef, who was convicted later of plotting the 1993 World Trade Center bombing.

Project Bojinka As part of "Project Bojinka" -- Serbo-Croatian for "loud bang" -- Murad was to crash a light aircraft loaded with explosives into CIA headquarters at Langley, he later told investigators. A U.S. government prosecutor described the entire plot as "one of the most hideous crimes anyone ever conceived" and said if executed to lethal perfection, it would have killed 4,000 people.

Murad's arrest came 13 days after four members of an Algerian terrorist group linked to al Qaeda hijacked an Air France flight as it prepared to leave Algeria for Paris. French authorities learned that the men planned to crash the plane into a Paris landmark such as the Eiffel Tower; commandos killed the hijackers during a refueling stop before the suicide plot could be carried out.

Asked whether the Aug. 6 intelligence briefing with the president had included references to the CIA plot or the Eiffel Tower, Rice said: "We knew that they had thought about hijackings in a number of places. But, again, the information . . . was not about those activities."

Two senior counterterrorism officials said U.S. intelligence and law enforcement agencies had more warning than Rice acknowledged that al Qaeda might use a hijacked aircraft as a weapon. One warning may have come as recently as last July during security precautions for the Genoa summit of the Group of Eight industrial powers, which Bush attended.

According to Italy's Deputy Prime Minister Gianfranco Fini, in remarks reported by the Italian news agency ANSA, Italy told the Americans "that there was the possibility of an attack against the U.S. president using an airliner. That's why we closed the airspace and installed the [antiaircraft] missiles" around the meeting site.

One knowledgeable U.S. official said there was strong intelligence suggesting al Qaeda's intention to attack Bush at the summit. There were ambiguous reports, the official said, suggesting the possibility of attack by air or sea.

Because of the threat from jetliners, it has been standard operating procedure since the Atlanta Olympics in 1996 to create "no-fly zones" for high-profile occasions designated "National Security Special Events." Others included the 50th anniversary summits of NATO and the United Nations. The no-fly zones are areas of restricted airspace defended by fighter jets and antiaircraft batteries.

Protection From Meteorites In the spring of 1998, Gale, of the University of Pennsylvania, was part of a three-man group that presented a terrorism analysis to FAA security officials. The analysis, he said, described two scenarios: one in which terrorists crashed planes into nuclear power plants along the East Coast; another in which they commandeered Federal Express cargo planes and crashed them into the World Trade Center, the Pentagon, the White House, the Capitol, the Sears Tower and the Golden Gate Bridge. Gale said the analysis was based in part on the Eiffel Tower threat and a small plane that had crashed onto the White House grounds in 1994.

"You can't protect yourself from meteorites," an FAA official responded, according to Gale. "He was saying it's too hard."

The next year, a report prepared for the National Intelligence Council, an interagency group, by the Federal Research Division, which is part of the Library of Congress, was titled "The Sociology and Psychology of Terrorism: Who Becomes a Terrorist and Why?" In a section on new terrorist threats, the report noted the Philippines plot and warned: "Suicide bomber(s) belonging to al-Qaida's Martyrdom Battalion could crash-land an aircraft packed with high explosives (C-4 and semtex) into the Pentagon, the headquarters of the Central Intelligence Agency (CIA), or the White House."

Flight Schools Probed The furor over the government's handling of intelligence before the Sept. 11 attacks was touched off by disclosure of a classified July memo in which a Phoenix FBI agent warned that bin Laden might be using U.S. aviation schools to train terrorists as pilots. Three days after the attacks, FBI Director Robert S. Mueller III described reports that several of the hijackers had received flight training in the United States as "news, quite obviously." He added: "If we had understood that to be the case, we would have -- perhaps one could have averted this."

But after Murad's arrest in 1995, FBI agents seized records and conducted interviews at flight schools he attended in New Bern, N.C., and Schenectady, N.Y., instructors at both flight schools told The Washington Post last September.

In addition, in 1998, the FBI visited Airman Flight School of Norman, Okla., to inquire about another al Qaeda operative, Ihab Ali Nawawi. Dale Davis, the flight school's director of operations, told The Post last year that Nawawi received his commercial pilot's license in the early 1990s, then traveled to another school in Oklahoma City to qualify for a rating to fly small business aircraft.

During last year's trial on the 1998 bombings of U.S. embassies in Africa, a witness testified that Nawawi later worked as bin Laden's personal pilot.

Two weeks before the Sept. 11 attacks, the FBI was back to interview Davis and his colleagues about another Airman student, Zacarias Moussaoui, who had taken more than 50 hours of flying lessons before flunking out. Moussaoui later traveled to a flight school in Minnesota, where he aroused suspicions and was arrested. He is being held on charges of conspiring with the 19 hijackers to destroy the World Trade Center and the Pentagon.

The Egyptian witness in the embassy bombing trial, Essam al Ridi, also attended flight school -- the now-defunct Ed Boardman Aviation School in Fort Worth. Al Ridi testified that in 1993 he bought a used Saber-40 aircraft for $210,000 for bin Laden, who wanted the plane to transport Stinger missiles, then flew it from Dallas-Fort Worth International Airport to Khartoum, Sudan.

Upon his arrival, al Ridi testified, bin Laden threw a dinner for him at one of his guest houses. Present were a number of bin Laden's bodyguards toting AK-47 assault rifles.

"We just had dinner and chatted and . . . I gave the keys of the airplane to Osama bin Laden," al Ridi testified.

Staff writer Barton Gellman and research editor Margot Williams contributed to this report.