The Central Intelligence Agency (CIA) has tweeted a declassified document that claims responsibility for over one half of all UFO reports made during the late 1950s and 1960s.

The document, titled The CIA and the U-2 Programme, 1954-1974, was the most read item on the Agency’s ‘Best of 2014 List’ and details flight testing of the Lockheed U-2 aircraft at higher altitudes than would have been normal for planes at the time.

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

#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

“In the mid-1950s, most commercial airliners flew at altitudes between 10,000 and 20,000 feet and military aircraft like the B-47s and B-57s operated at altitudes below 40,000 feet.

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

The CIA attributes the sun reflecting the “silver bodies” of the U-2 to the shapes and glints in the sky that were reported by people on the ground and by commercial pilots flying at lower altitudes.

“At this time, no one believed manned flight was possible above 60,000 feet, so no one expected to see an object so high in the sky,” it says.

The U-2 Programme was developed by the US government in the 1950s to test the potential of using high-flying aircraft for reconnaissance missions.