1. The Mastermind, the Hijackers and the Planes
The mastermind of the attacks was Osama Bin Laden, a rich Arab who ran a terrorist training school in Afghanistan, and, curiously, was at one time a 'client' of the CIA. The reason why Osama and his gang carried out the attacks is because they hate the freedoms of America.
The hijackers had learned to fly 757's using flight simulators. On the morning of 11 September, 2001 they boarded 4 planes as passengers in groups of four or five. After take off, they brandished small disposable stanley knives (sometimes called box cutters) which they had smuggled on board. They may have also used mace or pepper spray. Thus armed, they quickly gained complete control of the aircraft. On several of the planes they told passengers they had a bomb. We know all this because lots of passengers on the hijacked planes made calls from their cell phones. It is interesting that the hijackers, who ruthlessly prevented the pilots from contacting the control towers, nonetheless allowed passengers to make many phone calls. Also, it is a little surprising that so many calls got through, since it is normally difficult to make a cell phone call from a fast moving jet plane.
Unfortunately, neither the pilots nor co-pilots of any of the hijacked planes keyed '7500', the four-digit universal code for hijack, into their SSR transponders, an action which would have instantly alerted the FAA that a hijack was in progress. Nor did they use their radios to broadcast the voice message 'squawk 7-5-0-0', which would have had the same effect (Report, p.17). Instead, each flight crew surrendered its cockpit to the hijackers without a squeak (or squawk) to the outside world.
Curiously, one of the first things the hijackers did after commandeering each plane was to turn the transponders off. Under FAA guidlines this action itself would immediately trigger a hijack alert as shown in this FAA protocol from July 2001:
10-2-6. HIJACKED AIRCRAFT
When you observe a Mode 3/A Code 7500, an unexplained loss of beacon code, change in direction of flight or altitude, and/or a loss of communications, notify supervisory personnel immediately.
It seems strange and foolish that the hijackers switched off the transponders when this would only heighten the suspicions of Flight Control, who would then have to notify NORAD, resulting in the scrambling of intercept fighters. On this day, however, the hijackers were lucky: NORAD was alerted by air traffic controllers, but they weren't overly hasty in scrambling fighters. This might have had something to do with all the war games they were running that morning. By an incredible coincidence, one of these war games was a drill involving real planes acting as simulated hijacked planes, while another involved a scenario in which a plane crashed into a building.
Wielding their little retractable stanley knives, the hijackers gained access to the cockpits, which are normally locked (report p.85), neutralised the pilots and first officers and disabled the transponders before anyone had a chance to send out a hijack alert - and they did this on four separate airliners. The question, how did they do it is of primary importance for our understanding of 9/11 and for developing strategies to prevent this kind of thing in the future. It was carefully considered for three years by the team of experts who prepared the 9/11 Commssion Report. Since this issue is so important , I will quote all of the report's weighty conclusions:
Flight 11 - 'We do not know exactly how the hijackers gained access to the cockpit' (p.5).
Flight 175 - 'Both pilots had been killed' (p.7).
Flight 77 - 'the hijackers used knives ... and moved all the passengers (and possibly crew) to the rear of the aircraft' (p.8).
Flight 93 - In a radio transmission the captain or first officer could be heard shouting: 'Hey get out of here - get out of here - get out of here' (p.11).