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The aircraft – usually launched at the outbreak of a nuclear attack - was spotted circling by thousands of US citizens on the ground. The unbranded plane travelled across several states and changed directions numerous times, prompting a Twitter frenzy of conspiracy theories. These were only intensified after a spokeswoman for the Naval Air Forces confirmed the plane was in fact their own E-6B Mercury, also known as the Doomsday Plane.

TWITTER; GETTY The aircraft is launched at the outbreak of a nuclear attack

The plane took off from close to San Diego and veered towards the Pacific Ocean before turning around to circle over Denver in Colorado for an hour The £100 million aircraft eventually landed in Oklahoma several hours later. The US army owns 16 so-called 'doomsday planes', a powerful aircraft that holds the capability to launch nuclear missiles The jet becomes a flying command centre used in the event of a nuclear war.

TWITTER The flight circled Denver for an hour - and no-one knows why

GETTY The US army owns 16 so-called 'doomsday planes'

Unsolved Mysteries of the World Sun, January 8, 2017 Express Pictures reports on the world's unsolved mysteries. Play slideshow Getty Images 1 of 10 Loch Ness monster: There have been many sightings over the years and photos and videos of actual footage have been checked and looked at time and time again, confirming if it could be some kind of sea serpent or a descendant of the dinosaurs.

There was this plane just circling the metro, circling and circling, and many of you called us asking what was going on Broadcaster in Denver

One TV anchor in Denver, Colorado brought up the strange sight during a broadcast. She said: "There was this plane just circling the metro, circling and circling, and many of you called us asking what was going on." The newscaster added that Federal Aviation Administration (FAA), U.S. Northern Command (NORTHCOM), U.S. Strategic Command (STRATCOM) and the North American Aerospace Defense Command (NORAD) could not confirm the flight.

GETTY The unbranded plane travelled across several states and changed directions numerous times