Related: Pentagon has $22 million program to investigate UFOs
In the 1960s, Project Blue Book was an Air Force program that investigated worldwide reports of unidentified flying objects. The investigations, which concluded in 1969, found no threats to national security or evidence of extraterrestrial vehicles, the Air Force has said.
The Advanced Aviation Threat Identification Program inside the Pentagon ended around 2012, according to the Defense Department.
The $22 million program ran between 2007 to 2012, The New York Times reported late last year. At the time, the Pentagon released two videos of an encounter between Navy F/A-18s and unidentified aircraft off the West Coast in 2004.
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To The Stars Academy of Arts and Sciences, a private scientific research organization, released the latest Defense Department declassified video of the 2015 incident, USA Today reported.
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Christopher Mellon, a former deputy assistant secretary of defense for intelligence in two prior presidential administrations and an adviser to the To the Stars Academy for Arts and Science, called on the Pentagon to take the reports seriously and investigate the cause of the sightings.
"If the origin of these aircraft is a mystery, so is the paralysis of the U.S. government in the face of such evidence," he wrote in a Washington Post Op-Ed column Friday.