Right out of the book On Information Awareness:

1) Consider something that doesn't officially exist as a matter of "national security".
2) Allow no media coverage.
3) Government policy denies and and all study of aforementioned subject.
4) Any deviation is of this policy is ABSOLUTELY FORBIDDEN.

SOM1-01 (Majestic Special Operations Manual), April 1954

Any encounter with entities known to be of extraterrestrial origin is to be considered a matter of national security and therefore classified TOP SECRET. Under no circumstances is the general public or the public press to warn of the existences of these entities. The official government policy is that such creatures do not exist, and that no agency of the federal government is now engaged in any study of extraterrestrials or their artifacts. Any deviation from this stated policy is absolutely forbidden.

SOM1-01 (Majestic Special Operations Manual), April 1954