Again, the amount of people who would have had to have been involved in this conspiracy is astronomical - they would never be able to keep it under wraps.
So how many Top Secret or higher bits of data do you suppose exist, how much information is hidden due to "National Security" concerns, how many members of the US government know these data, and how much of it do you and I know? You act as if threatening to assassinate (and now, torture first) people and their familiies isn't a sufficient incentive to keep one's mouth shut.
Oh, and, by the way, they haven't kept it under wraps. The holes in the story demonstrate fairly easily that we're not being told the truth.