activity in the skies in the '50s? It was us,' the government agency tweeted yesterday

'#1 most read on our #Bestof2014 list: Reports of

The CIA announced on Monday that they are responsible for at least half of the UFO sightings reported in the 1950's and 60's.

'#1 most read on our #Bestof2014 list: Reports of unusual activity in the skies in the '50s? It was us,' the government agency tweeted yesterday.

Over the last few days, the CIA has been tweeting out links to its top ten most-read 'Best of 2014' documents.

The CIA explained that the UFO sightings of the past were were directly correlated to them testing U-2 spy planes at altitudes of over 60,000 feet, once thought an impossible height to fly.

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Tweet: '#1 most read on our #Bestof2014 list: Reports of unusual activity in the skies in the '50s? It was us,' the government agency tweeted yesterday

UPI reports that the CIA soon realized that UFO sightings lined up with the place and time the U-2 planes were flying but that they purposefully chose not to let people know that what they were seeing were not aliens but spies.

Now U-2 planes are flown by the United States Air Force and 104 have been built in total.

In 1960, the Soviet Union shot down a plane at 70,500 feet that was trying to enter the Soviet Union by going into Pakistan's air space then Norway's air space.

'High-altitude testing of the U-2…led to an unexpected side effect — a tremendous increase in reports of unidentified flying objects,' Venture Beat reports the CIA said in 1998.

'In the mid-1950s, most commercial airliners flew at altitudes between 10,000 and 20,000 feet and [many] military aircraft…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 document continues.

According to an article from The New York Times written in 1997, the U-2 planes were developed to photograph enemy targets from secret bases in California and Nevada.

The planes flew across the country and eventually overseas to bases in countries that included Britain, West Germany and Taiwan.

The spy planes never ventured towards nor did they come from outer space.