The idea that terrorists would hijack a commercial airliner and crash it into a large building, was not unknown to the anti-terrorism community before September 11, 2001. But no one ever thought the bad guys would be good enough (or lucky enough, don't underestimate that) to grab four airliners at once, much less one. Not everyone in the anti-terrorism business agreed with that, but no precautions were ever taken (like checking out the pilot training business to see if it was possible for foreign students to learn enough in a short period of time to be capable suicide pilots.) However, many anti-terrorist experts still consider it quite possible that a smaller aircraft could be used for suicide attacks. This scenario has terrorists renting a small two engine aircraft (like the Piper Aztec or Cessna Businessliner) and flying off to a target within several hundred miles of its takeoff. These aircraft rent for about $250 an hour (with a 3-4 hour minimum). They have a cargo capacity of about half a ton, and that could be filled with explosives. This would give the terrorists the equivalent of an American cruise missile (which has a one ton warhead.) These aircraft have a maximum take off weight of about three tons and only carry about 500 pounds of fuel. Probably would not bring down a large skyscraper, but would do a lot of damage to the White House or most other government buildings in Washington. You can buy these aircraft second hand for $200-300,000. And then there's the criminal underground angle. The drug smuggling business uses a lot these aircraft to move their merchandise. Terrorists often work with criminal gangs to obtain false identifications, weapons and other illegal services. At the moment, legitimate aircraft rental operations are on their toes, and any foreigners seeking to rent a twin engine aircraft would be carefully scrutinized. But al Qaeda's top operatives (and hundreds of these guys are unaccounted for) were trained in the techniques of working around such obstacles. Latin America has a lot of aircraft and weapons suppliers who deal with local rebels and drug gangs. If you have the money, you can get what you want and fly north. It's not quite as easy as it sounds, but it is possible.