Commercial airliners have a lot of controls, so you need specialized skills to fly them. Some of the September 11 hijackers took flying lessons in America.

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2000 -- al-Qaida in America

Bin Laden helps choose the lead hijacker, an Egyptian named Mohamed Atta. In 2000, Atta travels to the United States. Like the 18 other terrorists, he begins assimilating into society, attempting to avoid drawing undue attention to himself. Like a few of the other terrorists, Atta begins taking flying lessons, including a stint at an accelerated training program located in Florida. In 2001, the rest of the hijackers begin arriving in America. These secondary terrorists aren't slated to fly the hijacked planes. Instead, they plan to back up the primary pilot-hijackers and help them achieve their ultimate goals. The terrorists live scattered around the country, waiting for the cue to begin their operation.