The government has to know that the emergence of incontrovertible truth that proves such an event were to ever happen would result in the 2nd American Civil war at its worst, or such an extreme public outrage as to cause our entire government to be purged and replaced at the minimum.
This is the primary reason that I do not jump on the cover-up bandwagons that seem to pop up every few months. The people who would be responsible for doing this are not stupid and the sheer number of people who would have to be involved would guarantee its being proven very quickly and with no doubts as to its truth.
The day I no longer trust the government to serve the people, is the day I begin looking at Canada as my adopted home.