Well, I think none of us can say with certanity about 9/11, maybe it was a plane, but maybe the goverment was behind it or had a hand in it. Or maybe is really was a terrorist attack... Because after all there is always doubt...
But I don't think it would be in country's best intrest to lets say prove with some real evidence that the goverment was behind it. Its best to keep this story the way it is, even if its not true. I think it would cause choas, probably riots, severe mistrust of the government, probably economic chaos if such evidence were to come out.
And lets say the government was behind 9/11... In that case, there probably would be a very good reason to kill 3,000 U.S. citizens.... one that would perhaps benefit the nation 15, 20 years down the road , lets say when oil runs out or something... I don't know... I mean theories say that Pearl Harbour was known about.. well look what WW2 did for the country...