MYSTERIOUS sightings of large drones swarming in formation over America's Midwest have left cops baffled.

Up to 30 of the unmanned aircraft - each 6ft across - have been buzzing homes in rural Colorado and Nebraska almost every evening since before Christmas.

Mysterious swarms of drones flying in formation have been spotted in the US Midwest Credit: NBC

No one knows where they came from or what they are doing Credit: NBC

The unmarked silver and white drones have 6ft wingspans, witnesses say (file image) Credit: Getty - Contributor

They appear in grid formation with blinking lights around 150ft-200ft up and usually fly between 7pm and 10pm.

Police have warned it may be illegal to shoot them down after dozens of calls from concerned residents.

Government agencies and the US Army have said the drones are not theirs, and even Amazon denied it is responsible.

Now the Federal Aviation Administration has launched an investigation hoping to trace who is controlling them.

Missy Blackman, who saw three drones over her farm near Palisade, Nebraska, told the New York Times: “It’s creepy.

“I have a lot of questions of why and what are they, and nobody seems to have any answers.”

Some have suggested the drones could be mapping the landscape looking for oil or gas - but others say that is impossible in the dark.

Only once have they been seen in daylight. A witness who peered through binoculars said they were silver and white with no identifying markings.

SPYING FEARS

There is no suggestion the drone operators are breaking the law, but many fear their privacy is being invaded.

Dawn George, whose home has been buzzed more than once near Wray, Colorado, said she has heard wild theories about the flights.

She said: “All the sudden, it’s just going to stop and we’re not going to have answers.

“And that’s very unsettling to a lot of people. It’s the fear of the unknown.”

She added: “They’re high enough where you couldn’t shoot one anyway, but they’re low enough that they’re a nuisance.”

Sheriffs and officials in at least five counties are working together to track flight paths.

And Colorado senator Cory Gardner said he is in touch with the FAA and will "closely monitor the situation".

The drones have been spotted in five counties in Colorado and also across the border in Nebraska Credit: NBC

Sheriffs and the FAA are investigating who is controlling them Credit: NBC

One of those keen to know more is sheriff's captain Michael Yowell, who saw a drone cluster over his house in Lincoln County, Colorado, on New Year's Eve.

He told the NY Times: “We want to know, at around 10pm when we start to lose visuals, which direction are they homing? Which way are they heading?

“We hope that’s how we can contact somebody on the ground.”

Yuma County Sheriff Todd Combs said the drones are flying in air space controlled by the federal government, but federal agencies have no knowledge of them.

He said: “There are many theories about what is going on, but at this point, that’s all they are.

“I think we are all feeling a little bit vulnerable due to the intrusion of our privacy that we enjoy in our rural community, but I don’t have a solution or know of one right now.”

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Amazon, Paragon Geophysical Services, the US Geological Survey, the Colorado Department of Transportation and UAV Recon have all denied involvement, according to the Denver Post.

All but the smallest drones have to be registered with the FAA, but at present there is no way of tracking them.

In May the Department of Homeland Security warned Chinese-made drones could be harvesting top secret data and sending it back to spies in Beijing.