One thing I ran across recently (heard on TV IIRC) was a transcript of the actual communications between the air traffic controllers and the air force. In that there was the question asked by the military "Is this a drill". The response from ATC was "No, no drill".
But there is little need for any conspiricy theory IMHO. Afterall, the subject had been the primary plot line of a best selling novel just a few years earlier. In addition, there was the known attempt to fly a jumbojet into the Eiffle Tower.
There was certainly a failure in intellegence. The question is whether or not it was an avoidable failure. I imagine that any one of us can outline a hundred or so terrorist scenarios and many are quite likely to come true.
The issue of ubiquitous terrorism is a new paradigm, one that has significantly changed the status quo. So far no one really has worked out the method of dealing with it but IMHO the best model to follow will be based on criminal pursuit of organized crime rather on the military intervention model.
Aslan is not a Tame Lion