NORAD didn’t do it’s job
Prison Planet - Why didn't NORAD fulfil its standard operating procedure and intercept the planes? Were NORAD intentionally confused by the wargames taking place on the morning of 9/11 or were they ordered to stand down?
FromTheWilderness - The Air Force was running multiple war games on the morning of 9/11 simulating hijackings over the continental United States that included (at least) one "live-fly" exercise as well as simulations that placed "false blips" on FAA radar screens.
911Research - Routine interception procedures were not followed on September 11th, 2001.
The 9/11 Stand Down - 9/11 Timelines - the key to understanding why the Trillion Dollar Air Force did not defend its headquarters
William Thomas - When will someone ask why America’s air defenses were stood down on Sept. 11?
Stand Down - Exposing NORAD's Wag The 911 Window Dressing Tale
National Security Archive - these documents contain minute-by-minute accounts of unfolding events as experienced by FAA officials, including radar reports and extensive chronologies tracking the larger U.S. government response to the attacks from September into late October 2001.
Government Releases Detailed Information on 9/11 Crashes - The National Transportation Safety Board (NTSB) this week released full transcripts of the air traffic control recordings from the four flights hijacked on September 11, 2001, and meticulous Flight Path Studies for three of the flights, in response to a Freedom of Information request by the National Security Archive. The studies provide the most detailed technical information available to date related to the hijackings, and the transcripts of the aircraft-to-ground communications are the first complete government disclosure of each flight's air traffic control recordings.