In 1947 there were so many UFOs spotted by civilians and pilots that the U. S. military became concerned, wondering if the Soviet Union had somehow managed to leap frog their technology and were able to enter U.S. air space undetected. In response they created Project Sign. Project Sign's primary task was to gather data, analyze it and report a recommendation.
They did this, and presumably in a top secret memo, informed the high command that there was credible evidence that aliens were poking around the atmosphere. The High Command promptly shut down Project Sign and replaced it with Project Grudge. Grudge had a different crew, one made up of less alien-minded people (whereas Sign had some alien-adherents in their ranks) and came to the exact opposite conclusions, stating there was no credible evidence.
They were shut down to, admittedly at their own recommendation. This led to some accusations that Grudge was fixed, so the USAF launched Project Blue Book.
Blue Book was supposed to gather data and carefully analyze it. Though the data was gathered there was some question as to their analysis of it and man accusations that it was, much like Grudge, a farce.
But you can read all that for yourselves over at NICAP (National Investigatons Committee for Areal Phenomena) website.
Read Projects Sign, Grudge and Blue Book.