One thing that makes me discount the WMD theory is that if all they want is an aircraft as a weapon, then stealing one with 239 people on board is a high risk, high profile strategy. If you have the capability and resources to steal the plane, store it, and re-equip it, then you would have the resources to buy or lease one. There are plenty of 767's out there that could be obtained cheaply and without drawing attention to yourself. If you are just going to stuff it full of explosives and fly it at something, then you don't need fuel efficiency and advanced avionics. You could quietly procure one or more 767's and catch everyone by surprise, whereas now the whole world is on alert. These people are not stupid, so am I missing something here?