As far as "best of 2014" lists go, the CIA has a pretty irresistible one: On Dec. 22 it started tweeting links to the 10 most popular articles of the year that it shared on Twitter, and the agency arrived at No. 1 yesterday, tweeting: "Reports of unusual activity in the skies in the '50s? It was us." The accompanying link directs readers to The CIA and the U-2 Program, 1954-1974, a 272-page document from 1998 the CIA tweeted a link to in early July, reported KAKE at the time.

The upshot of the report is that the CIA was the culprit behind more than half of the UFO sightings logged in the 1950s and 1960s.

As VentureBeat reports, the CIA tested its U-2 spy planes at 60,000 feet, an altitude that seemed impossible for man to reach at the time—leading observers, specifically pilots, to suspect it wasn't man up there at all.

VentureBeat highlights a portion of the report that explains that in the mid-1950s, most commercial airliners stuck below 20,000 feet; military aircraft kept it 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 actually cross-checked the UFO reports with its flight records, it noted in the document, but in instances when it verified the UFO was really a U-2, it stayed mum.

The report was part of documents declassified in 2013 that famously detailed the existence of Area 51 in Nevada. As for the rest of the CIA's top 10, it includes a look at a day in the life of a "not yet burned out" CIA Operations Center Officer and a confirmation that pigeon missions remain classified.

(More on its Area 51 revelations here.)

This article originally appeared on Newser: CIA: All Those 1950s UFO Sightings? 'It Was Us'

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