The U.S. Air Force invented the term “unidentified flying object” in 1952 to replace the term “flying saucer.”  The Air Force defined a “UFO” as “any airborne object which by performance, aerodynamic characteristics, or unusual features does not conform to any presently known aircraft or missile type, or which cannot be positively identified as a familiar object.” [1]
Every UFO sighting report can be placed into one of three categories:

1. Identified Flying Objects (IFOs): Those reports that are investigated and determined to be known objects.  These can include astronomical phenomena, aircraft, balloons, advertising planes, experimental aircraft, unmanned aerial vehicles, flocks of birds, and hoaxes.
2. Insufficient Data: Reports that don’t provide enough data to reach a conclusion.
3. Unknowns: These are reports by competent observers of objects which cannot be identified by the witness, and which remain unidentified after investigation by competent investigators, and whose appearance indicates that they were manufactured, and whose flight behavior indicates that they were made somewhere other than Earth. 

Over a period of years, a number of classified documents detailing Operation Majestic 12 were leaked to multiple sources.  Some of the documents have been shown to almost certainly be inauthentic.  Others, however, seem to have passed rigorous scrutiny.
The documents indicate that Operation Majestic 12 was established by special classified presidential order on September 24, 1947.  The goal of the group was to exploit everything they could from recovered alien technology. [1]
According to Stanton Friedman, who has been researching these documents since 1984:

In addition to an entire book devoted to this subject, Friedman dedicates a chapter in his book,Flying Saucers and Science: A Scientist Investigates the Mysteries …, to explain in detail why these particular documents appear to be authentic, and why others don’t.  Friedmen has spent countless hours visiting archives, talking with experts, reviewing flight logs, and digging for details.  The government cover-up amounts to what he refers to as a “Cosmic Watergate.”

The U.S. government has successfully kept many big secrets over long time-spans.  Consider these as examples:

• The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. The Manhattan Project began modestly in..SEE– http://alien-ufo-community.ning.com/forum/topics/are-aliens-real