Residents of rural Colorado have been left “very nervous and anxious” by a group of unmanned drones that have been gathering in the sky over a period of weeks.

At least 17 of the drones have been spotted looming above north-east Colorado and south-west Nebraska. In the week before Christmas the unmarked drones appeared over Phillips county every night at 7pm.

According to local law enforcement, the drones fly in harmony across the evening sky.

“They’ve been doing a grid search, a grid pattern,” the Phillips county sheriff, Thomas Elliott, told the Denver Post. “They fly one square and then they fly another square.”

On other occasions, the drones have split up. The Denver Post reported that one evening in late December eight of the drones were spotted flying in a straight line in Yuma county, which neighbors Phillips county.

“At the same time, a single drone hovered about 25 miles away over the town of Paoli – it didn’t move all night, [it] just hovered over the town,” according to the Post.

The Federal Aviation Administration (FAA) is now investigating the behaviour of the drones – which are said to be larger than a typical model aircraft, with 6ft wingspans – while local law enforcement have been at a loss to explain the drones’ existence and behavior.

“These drones have made residents in our community very nervous and anxious. People do not like the unknown as it upsets the balance of our lives,” the Yuma county sheriff, Todd Combs, said in a Facebook post.

In a separate post, the Yuma county sheriff’s office said it believed the appearances of drones “are not malicious in nature”, but suggested residents contact the local sheriff’s office should they witness any drones “looking into windows”.

One theory as to the drones’ emergence and behaviour is that is that they are being operated by a private company or government department. The next step in the drone investigation is a 6 January meeting between sheriffs, police and an FAA inspector, which investigators hope will shed some light on the mysterious night-time visitors.

In the meantime, locals have been advised not to shoot the drones because their batteries are flammable.

“It becomes a self-generating fire that burns until it burns itself out,” the drone pilot Vic Moss told the Denver Post. “If you shoot a drone down over your house and it lands on your house, you might not have a house in 45 minutes.”