The Sept. 11 commission's final report outlined this series of "operational failures" by the government that allowed the terrorists to carry out their plot.

— Failing to include hijackers Khalid al-Mihdhar and Nawaf al-Hazmi on a watch list until late August 2001. The two were linked by the CIA to Al Qaeda (search) and were known to have entered the United States in the summer of 2001, but government agents lost track of them. The two would later board American Airlines Flight 77, which slammed into the Pentagon.

— Not sharing information linking individuals in the October 2000 bombing of the USS Cole (search) in Yemen to al-Mihdhar, who had contacts with a longtime FBI informant. The Cole attack killed 17 American sailors.

— Not taking adequate steps in time to find al-Mihdhar and al-Hazmi in the United States.

— Not linking the August 2001 arrest of Zacarias Moussaoui (search) to the threat of an upcoming attack. Moussaoui was arrested after arousing suspicions at a Minnesota flight school. He has admitted belonging to Al Qaeda but denied he was part of the plot.

— Not discovering false statements on visa applications of some of the future hijackers.

— Not recognizing that some hijackers' passports were fraudulent.

— Failing to expand no-fly lists to include names from terrorist watch lists.

— Not searching airline passengers identified by a computer-based airline screening system known as the Computer-Assisted Passenger Pre-screening System.

— Not hardening aircraft cockpit doors or taking other measures to prepare for the possibility of suicide hijackings.