They come in the night: Drones — lots of them — flying in precise formations over the Colorado and Nebraska prairie.

Whose are they? Unknown.

Why are they there? Unclear.

“It’s creepy,” said Missy Blackman, who saw three drones hovering over her farm outside Palisade, Nebraska, on a recent evening, including one that lingered right above her house. “I have a lot of questions of why and what are they, and nobody seems to have any answers.”

Since before Christmas, sheriff’s departments in the region have been bombarded with reports of large drones with blinking lights and wingspans of up to 6 feet flying over rural towns and open fields. The drones have unnerved residents, prompted a federal investigation and made international news, even though they may be perfectly legal. And still, they remain unexplained.

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“In terms of aircraft flying at night and not being identified, this is a first for me personally,” said sheriff James Brueggeman of Perkins County, Nebraska, who has worked in law enforcement for about 28 years and who saw the drones while on patrol Tuesday night.

He said he had heard rumblings about people wanting to shoot down a drone, and had urged residents to report the sightings to law enforcement instead. “I think it’s kind of a joke, but you have to remember the part of the country we live in,” the sheriff said. “People here don’t like their privacy to be invaded.”

The flights have drawn attention just as the Federal Aviation Administration (FAA) last week proposed sweeping new regulations that would require most drones to be identifiable. Ian Gregor, a spokesman for the FAA, said that the timing of the proposed rule was coincidental, but that the agency had opened an investigation of the sightings in Colorado and Nebraska.

“Multiple FAA divisions and government agencies are investigating these reports,” Mr Gregor said in an email. He declined to discuss the inquiry in detail, but said investigators were trying to determine who was operating the drones and the purpose of the flights.

On Facebook, 911 dispatch lines and local newspaper columns, the drones have been the talk of rural Colorado and Nebraska. And as sightings increase — people in four counties said they had seen them on Tuesday — so too does the urgency of residents’ questions.

Some have suggested they might be part of a simple mapping operation, or a land survey conducted by an oil and gas company — but why would such flights run at night?

Senator Cory Gardner said on social media that he would “closely monitor the situation.” A newspaper headline in Akron, Colorado, asked, “What’s with the drones?” Multiple law enforcement agencies warned residents that shooting a drone out of the sky would be a crime.

“They’re high enough where you couldn’t shoot one anyway, but they’re low enough that they’re a nuisance,” said Dawn George, who lives near Wray, Colorado, who said her border collie has barked at the drones when they fly over her property.

Ms George said she had heard wild speculation about who might be responsible for the flights — the government? A cartel? A gas company? — and feared they would never know the truth.

“All the sudden, it’s just going to stop and we’re not going to have answers,” Ms George said. “And that’s very unsettling to a lot of people. It’s the fear of the unknown.”

Unmanned drones, which have exploded into popular usage in recent years and can be used for everything from mapping to photography to farming, can be difficult to track. Operators of all but the smallest drones have been required to register with the federal government since 2015, but there is no straightforward, legal way for state and local officials to identify the owner of a particular drone or to track that drone’s location.

“Most people are very reasonable, and they say it could be somebody mapping or doing topography,” said Michael Yowell, a sheriff’s captain in Lincoln County, Colorado, whose house was buzzed over by a drone squadron on New Year’s Eve. “But you can’t rule out what you don’t know.”

The drone sightings started in northeast Colorado around mid-December and have only grown more widespread since then. Almost all the sightings have occurred between sunset and about 10pm, though Ms Blackman said she had seen them out later one night in Nebraska and, for the first time on Wednesday, during daylight hours. She said she had looked at them through binoculars and did not see any markings, just plain silver and white colouring.

Across the state line in Colorado, Mr Yowell tried to photograph the drones on Tuesday night with the camera he uses to document crime scenes, but came away without a clear image. He estimated that up to 30 drones were flying each night, though not all in the same place.

He said local officials were studying the flight path of the drones and coordinating across county lines to figure out where they were coming from. If his analysis was correct, he said, the drones would be back out on Wednesday night, flying in a grid pattern in the rural area between Hugo and Karval, Colorado, about 100 miles southeast of Denver.

“We want to know, at around 10 o’clock, when we start to lose visuals of these, which direction are they homing? Which way are they heading?” Mr Yowell said. “We hope that’s how we can contact somebody on the ground.”

Sheriff Todd Combs of Yuma County, Colorado, said in a Facebook post on Tuesday that the drones appeared to be staying at least 150 feet from buildings or people, based on the footage he has seen.

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