Compelling evidence, including video tape of hijackers entering through checkpoint screening stations, suggest that the hijackers gained access to the aircraft on September 11th through passenger checkpoints.

The 9/11 hijackers exploited the significantly more relaxed aviation security standards that existed prior to September 11, 2001. At that time, airport screening was handled by private security companies contracted by airlines, and many items now banned were legally permitted. Specifically, knives with blades shorter than 4 inches, including the box cutters used in the attacks, were allowed in carry-on luggage. Furthermore, the hijackers did not have to remove their shoes, belts, or outerwear, and they could bring unlimited quantities of liquids. On the morning of the attacks, several hijackers actually set off the metal detectors (including Khalid al-Mihdhar and Majed Moqed), but they were cleared after a cursory wanding because the items they were carrying did not violate the rules of the time. The FAA's "CAPPS" (Computer-Assisted Passenger Prescreening System) flagged six of the hijackers for additional screening, but since this only required their checked bags to be screened for explosives (not their persons), they were allowed to board with their lethal carry-on items.