How is that evident? Not a single frame of airport security footage was ever released.
I had thought that this was revealed during the phone conversations.
Several of the passengers who called their loved ones, on both cell phones (yes, they can be used in flight--this is discouraged only because it can interfere with ground to air and air to ground communication) and Airphones on the backs of the seats, described these hijackers as middle eastern looking (one said "Iranian looking"). They also described the killing of a first-class passenger, probably Mickey Rothenberg. This is the undeniable truth, unless you consider the passengers to be part of the "plot".