In my discussions with doubters of the view that 9/11 was an inside job, the argument they often raise is that it would have been impossible to keep such a vast conspiracy a secret. Inevitably, I ask them how vast they think it would need to have been. So far, none of them has given me any answer at all. You would think that at least one of them had come up with some sort of estimate. It turns out that an estimate was arrived at, but not by a doubter, but by a 9/11 Truther: Jim Hoffman, of 9-11 Research. Long ago he offered the hypothetical Attack Scenario 404: How the Attack Might Have Been Engineered (But Probably Wasn't), which would have taken about a dozen people. As he described it:
This scenario shows that, through their positions of access in the military command structure, a very small group of people would have been able to appropriate these technologies to carry out the attack. While the attack is engineered by a core of only a dozen people, vast numbers of people facilitate the attack and cover-up, for the most part unknowingly, by simply doing what they normally do in their positions: promote and protect their agencies and the status quo. The public at large participates in the cover-up by failing to question the attack and instead believing the relatively comforting myth of bin Laden."
If only a dozen people or so were really in the know, it doesn't seem at all implausible that it was kept a secret. In fact, I imagine two or three dozen people could have known, without leaks occuring.