9-11 Deniers love to claim that if the proper procedures have been followed, all four planes would have been shot down before they reached their targets. Of course, this would still have resulted in a 9-11 "Truth" Movement, just with different charges:
The airliners were shot down so that Ted Olsen could get out of his marriage! Todd Beamer had discovered something in the Oracle databases that indicated the Florida 2000 election should have been won by Gore!
Ad nauseum. But I've never really looked into the question of shooting down aircraft by the military. As it happens, there is a procedure, and the procedure as of June 2001 said that the military may not shoot down planes:
a. General. Military personel will provide the following types of support: intercept, surveillance, lift, equipment and communications. Military personnel may not participate in a search, seizure, arrest or similar activity. This restriction would include the apprehension of aircraft hijackers or use of military aircraft (fixed-wing or helicopter) or other vehicles as platforms for gunfire or the use of other weapons against suspected hijackers.
Of course there could be a shoot-down order, because the Commander in Chief could supersede these standing orders. But the notion that shooting down the planes should have been automatic procedure is nonsense, like most of the other stuff the 9-11 Deniers say.