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, Neb., 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.

“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, Neb., who has worked in law enforcement for about 28 years and who saw the drones while on patrol Tuesday night.