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.
July 6, 2001 - The National Security Council's Counterterrorism Security Group meets, concerned about the increased potential for attacks in Europe and the Mideast.
July 10, 2001 - FBI agent Kenneth Williams writes a memo about Arabs training at Phoenix area flight schools, speculating that they could be al-Qaida. He requests that a canvass of Middle Eastern men training at U.S. flight schools be done.
July 2001 - The CIA warns the White House of possible attacks against President George W. Bush during the July G-8 Summit in Genoa, Italy.
July 18, 2001 - The FAA issues another warning to airlines concerning possible terrorist attacks.
July 28, 2001 - Djamel Beghal is arrested in the United Arab Emirates for carrying a false passport. Under interrogation he details his role in al-Qaida and its plans to blow up the U.S. embassy in Paris.
July 31, 2001 - The FAA issues another warning to airlines advising that "terror groups are known to be planning and training for hijackings."
Aug. 6, 2001 - Bush receives a CIA report about al-Qaida and the possibility of airline hijackings. The warning is passed on to embassies and other overseas facilities.
Aug. 15, 2001 - The Pan Am International Flight Academy in Minnesota alerts the FBI to their suspicions about Zacarias Moussaoui. He had paid for the training in cash and requested instruction on flying large jets, even though he had little experience.
Aug. 16, 2001 - The FAA issues an alert about "disguised weapons." Airlines are alerted the terrorists might use common objects such as cell phones or clothespins as weapons.
Aug. 17, 2001 - Zacarias Moussaoui is arrested on charges of overstaying his 90-day visa. The FBI interrogates Moussaoui for two days before he requests a lawyer.
Aug. 23, 2001 - CIA director George Tenet, concerned that an al-Qaida attack is imminent, orders the CIA to search their files for leads. An urgent cable is sent to the FBI, State Department, Customs and INS, alerting them to the CIA's concerns about Nawaf Alhazmi and Khalid Almihdhar. After the INS reports that Almihdhar re-entered the country on July 4, the FBI begins searching for them.
Sept. 4, 2001 - The national security advisers to President George W. Bush approve a draft version of a plan to combat al-Qaida. It includes provisions for $200 million to arm the enemies of the Taliban. The advisers plan to present the draft to Bush on Sept. 10, however the president is traveling and does not see it.
Sept. 10, 2001 - The National Security Agency intercepts two communications from Afghanistan to Saudi Arabia. One says, "Tomorrow is zero hour," and the other says "The match begins tomorrow." The messages are not translated until Sept. 12.