Sept 11 hijackers 'twice set off security alarms'

Some of the hijackers who crashed a jetliner into the Pentagon on September 11, 2001, twice set off alarms as they passed through airport metal detectors, but security screeners did not appear to question them about knives they were apparently carrying, according to a newly disclosed surveillance video.

The video represents the only footage known to exist showing any of the September 11 hijackers boarding their final flights that fateful morning.