THE truth is out there.

On Twitter, that is.

America’s Central Intelligence Agency has used the end-of-the-year silly season to finally come clean about UFOs.

Anyone hoping for little green men will be disappointed, though — the CIA is claiming responsibility.

Tweeting about its most popular stories of the year, the CIA named a 1998 report linking its activities in the 1950s to UFO sightings.

#1 most read on our #Bestof2014 list: Reports of unusual activity in the skies in the '50s? It was us. http://t.co/BKr81M5OUN (PDF 9.26MB) — CIA (@CIA) December 29, 2014

The report, ‘The CIA and the U-2 Program, 1954-1974’, written by Gregory Pedlow and Donald Welzenbach, outlines the CIA’s involvement in the development of the U-2 spy plane.

It explains how the testing of the planes led to a massive increase in UFO reports.

“High-altitude testing of the U-2 soon led to an unexpected side effect — a tremendous increase in reports of unidentified flying objects (UFOs),” it says in a section devoted to the issue.

The reason? No, not swamp gas.

In the mid-1950s, most commercial airliners flew between 10,000 and 20,000 feet, while military aircraft such as B-47s and B57s flew below 40,000 feet.

“[UFO] reports were most prevalent in the early evening hours from pilots of airliners flying from east to west. When the sun dropped below the horizon of an airliner flying at 20,000 feet, the plane was in darkness,” the authors explain.

“But, if a U-2 was airborne in the vicinity of the airliner at the same time, its horizon from an altitude of 60,000 feet was considerably more distant, and, being so high in the sky, its silver wings would catch and reflect the rays of the sun and appear to the airliner pilot, 40,000 feet below, to be fiery objects.

“Consequently, once U-2s started flying at altitudes above 60,000 feet, air-traffic controllers began receiving increasing numbers of UFO reports.”

The report adds that at the time, no one believed manned flight was possible above 60,000 feet, and so didn’t expect to see objects so high in the sky.

The high volume of UFO sightings from airline pilots and ground-based observers writing to the Air Force led to the establishment of Operation Blue Book, which collected all reports for investigation.

According to the paper, Blue Book investigators regularly called on the CIA’s project staff in Washington to check reported UFO sightings against U-2 flight logs.

It says U-2 and later OXCART flights accounted for more than half of all UFO reports during the late 1950s and most of the 1960s, although investigators could not reveal to the letter writers the true cause of the sightings.