Here's a look at what you need to know about the warning signs that preceded the terrorist attacks of Sept. 11, 2001.
Dec. 24, 1994 - Four Algerians hijack an Air France plane in Algiers, Algeria.
Jan. 6, 1995 - Abdul Hakim Murad is arrested in Manila, Philippines. Murad details plans to blow up U.S airliners over the Pacific and to crash a plane packed with explosives into either CIA headquarters or another U.S. federal building.
May 18, 1998 - An FBI agent writes a memo about the number of Arab men seeking flight training in Oklahoma City, Okla. The memo is not sent to FBI headquarters.
Oct. 8, 1998 - The Federal Aviation Administration warns airlines and airports to maintain a "high degree of alertness." The warning is in response to statements made by Osama bin Laden in the aftermath of the U.S. bombings of al-Qaida targets in Afghanistan and Sudan.
Dec. 8, 1998 - The FAA warns of the possibility of a hijacking at an eastern U.S. airport.
Dec. 29, 1998 - The FAA issues a third warning. It restates the earlier warnings about threats made by Osama bin Laden.
1999 - French intelligence puts Zacarias Moussaoui on a watch list of suspected terrorists.
September 1999 - A report commissioned by the National Intelligence Council states that "Al-Qaida poses the most serious terrorist threat to U.S. security interests" and warns that al-Qaida "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."
December 1999 - The CIA intercepts phone conversations in Yemen detailing plans for an upcoming al-Qaida summit in Malaysia.
Dec. 14, 1999 - Ahmed Ressam is arrested in Washington state, trying to enter the U.S. from Canada. In his car, investigators find 130 pounds of bomb-making materials. It is eventually revealed that Ressam planned to blow up Los Angeles International Airport during celebrations for the Millennium.
January 2000 - Malaysian Intelligence monitors a meeting of suspected al-Qaida operatives in Kuala Lumpur. Malaysia shares surveillance photos and information with the CIA. The meeting takes place at a condo owned by Yazid Sufaat.
Jan. 15, 2000 - The CIA tracks Nawaf Alhazmi from the Malaysian summit to Los Angeles. The CIA does not alert the INS or the FBI of Alhazmi's entry into the U.S.
September 2000 - U.S. intelligence sources begin to intercept a series of threats against the U.S., mostly overseas. The intelligence "chatter" peaks in the summer of 2001.
October 2000 - The FBI arrives in Yemen to investigate the bombing of the USS Cole. In their investigation of suspect Tawfiq bin Attash, the FBI unearths photos of him taken at the Kuala Lumpur summit in January. In one photo he is shown standing next to Khalid Almihdhar.
2001 - Zacarias Moussaoui trains at the Airman Flight School in Norman, Okla. The school had been the subject of a FBI investigation in 1998.
Jan. 1-Sept. 11, 2001 - The FAA issues 15 information circulars containing generalized warnings about terrorist threats and hijackings.
Feb. 5-May 29, 2001 - Four defendants stand trial in New York for the 1998 embassy bombings. Prosecution witnesses describe buying airplanes and learning to fly them at the request of Osama bin Laden.
June 2001- Al Jazeera broadcasts videotaped threats against the West from bin Laden.
June 2001 - The State Department Consular Office in Saudi Arabia renews the expired visa of Khalid Almihdhar, unaware of the CIA's surveillance of him.
June 22, 2001 - The FAA issues an information circular with generalized warnings about threats to U.S. airlines.
June 26, 2001 - The State Department issues a worldwide caution concerning possible overseas terrorist activity.
July 5, 2001 - The FAA issues another warning, specifically the threat of "using explosives in an airport terminal."
July 5, 2001 - President George W. Bush requests more information about the recent upswing in terrorist threats and "chatter" from National Security Adviser Condoleezza Rice.