A plan to crash a plane into CIA headquarters was exposed after an arrest in the Philippines. A meeting of future Sept. 11 hijackers aroused suspicion in Malaysia. Information that al-Qaida was seeking to assassinate President Bush at a summit in Europe led to heightened security.
Clues filtering in from overseas since at least 1994 foreshadowed Osama bin Laden's plans to attack America, and intelligence information from Italy, Israel and elsewhere in the months before Sept. 11 warned that a terrorist strike might be imminent.
The White House acknowledged Thursday that President Bush was briefed by the CIA on Aug. 6 about an al-Qaida hijacking threat. An earlier report by the Phoenix office of the FBI that may never have reached the president's desk warned that many Middle Eastern men were training in at least one U.S. flight school.
The Bush administration said the information was not specific and there was no intelligence before Sept. 11 that al-Qaida planned to use commercial planes as vessels of destruction.
"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, a hijacked airplane as a missile," national security adviser Condoleezza Rice said Thursday.
Warning signs were there
But warning signs that something like Sept. 11 might be contemplated weren't all recent -- and they came from different sources around the world.
In 1994, Algerian militants hijacked an Air France jetliner and killed three passengers before being captured during a stop in Marseilles. It came out that they had hoped to blow up the jet over the Eiffel Tower, debunking the notion that a suicidal airline attack on a prominent target was unthinkable before Sept. 11.
Perhaps the first clue of a similar plot against the United States emerged during the Clinton administration in 1995, when Philippine authorities arrested Ramzi Yousef and Abdul Hakim Murad after a chemical fire at their Manila apartment.
Under questioning, Murad admitted connections to bin Laden and spoke of a plot to dive-bomb a jetliner into the CIA headquarters in Langley, Va. He also said Middle Eastern pilots were training at U.S. flight schools in preparation for a plot to blow up 12 passenger jets over the Pacific Ocean.
FBI checks flight schools
The FBI was alerted at the time and interviewed flight school attendees, but it did not develop evidence that any of the Middle Easterners were plotting terrorism.
Yousef, considered the mastermind of the 1993 World Trade Center bombing, and Murad were eventually convicted in the United States and sentenced to life in prison.
Another clue to Sept. 11 came in 2000, and it was partially a result of the 1995 Philippine investigation.
The investigation of Murad and Yousef led authorities to a radical Indonesian cleric, Riduan Isamuddin, who was living in Malaysia and was suspected of deep ties to al-Qaida and other terrorist organizations.
The cleric, who goes by the name Hambali, was under surveillance in January 2000 when he met with two future Sept. 11 hijackers -- Saudi nationals Khalid Almihdhar and Nawaf Alhazmi -- in Kuala Lumpur, Malaysian authorities have said.
Information from the surveillance was shared with U.S. authorities, and the meeting took on new significance when another of the participants, an unidentified al-Qaida operative from the Middle East, became wanted in connection with the October 2000 attack on the USS Cole.
The two Saudis were not on the intelligence community's radar screens at the time, but their connections to Hambali and the al-Qaida operative wanted in the Cole investigation got them placed on a CIA terrorist watch list in August 2001 -- one month before they helped commandeer the American Airlines jetliner that crashed into the Pentagon on Sept. 11.