All you need are about 16 fairly intelligent people, some knives, and Microsoft Flight Simulator.
I cannot fathom why people think this required state support at all. The planning was somewhat intricate but the mechanics and level of knowledge and research needed were not particularly sophisticated.
Of course, people don't WANT to believe that small autonomous groups are capable of this much destruction but when you look at the items and training necessary, you'll see that, at root, this was a fairly simple operation.
I have spent the better part of today listening to interviews of people who specialized in security, all of whom say the same thing. Unless you think there is a massive press conspiracy, what reason would they all have to lie?
I pretty much agree with your analysis. The tricky part would be to get ID to get on the flights with. Not really that big of a problem if you are talking about people already in the country, but if you have to import the principles from overseas, it gets a bit trickier and more expensive. My estimate was for 30 people and $250,000. Not that difficult. Of course, everything needs to be compartmentalized for security's sake.