I often wonder if they had full knowlege that they were going to hijack the planes but that the plan was to gain the political capital fom the attempt but to end it peacefully in some way (or at least without the death of passengers) and that it was an enormous surprise when they took control of the planes but rather than fly them, crashed them into buildings. If this was the case then you really can't claim that a '911 conspiracy' would be too big to hide and that somebody would have to let the cat out of the bag. Anybody and everybody who was part of that scheme would never mention a breath outside of their co-planners to anybody ever. Anybody who was even in the situation where they didn't know about it but in some way was responsible for letting others do it would probably keep quiet too. If they threatened to blab then I imagine that those responsible would just say that they were actually part of it but are trying to wriggle out and blame others to save their skin. If you thought that the American public were going to find out that you hatched a plan that went so famously and terribly wrong then you would do whatever it took to keep it from them.