Excerpt from usatoday.com

Maybe it was a . Maybe it was a plane.
But it was probably not a UFO.
The Central Intelligence Agency had some fun Monday tweeting out its most popular stories of the year.
No. 1? “Reports of unusual in the skies in the ’50s? It was us.”
That’s right – a CIA from 1998 was the most-read from 2014. It’s titled, innocuously enough, “The CIA and the U-2 , 1954-1974” is written by Gregory W. Pedlow and Donald E. Welzenbach.
And it has a whole section about .
The U-2, of course, is the CIA’s spy plane, and it made several high-altitude reconnaissance missions (and test flights) in the 1950s and 1960s. The document tracks the plane’s creation and iterations — and how it was often mistaken for a UFO because it flew higher than anyone believed was possible.
From the “U-2s, UFOs, and Blue Book” section of the lengthy report:
“High-altitude testing of the U-2 soon led to an unexpected effect — a tremendous increase in reports of (UFOs). In the mid-1950s, most commercial airliners flew at altitudes between 10,000 and 20,000 feet … Consequently, once U-2s started flying at altitudes above 60,000 feet, air-traffic controllers began receiving increasing of UFO reports.”
Reports were mostly made “in the early evening hours from pilots of airlines flying from east to .” After the sun dropped below the horizon, the U-2s would look like “fiery objects.”
Finally, the kicker: “… the flights accounted for more than one-half of all UFO reports during the late 1950s and most of the 1960s.”
And now you know.