I wonder if the hijackers were apart of a drug trafficking ring. Several of them flew in and out of the country, under the eyes of the INS, and the FBI, without ever being arrested, or stopped. Afghanistan was their destination for the most part, and we all know what Afghanistan is made up of. I wonder if "Al-Qaeda" has matured from mercenaries to drug runners.

That would explain why they were trained to fly at U.S. military installations.