A series of nighttime flights of what appears to be a drone swarm has northeastern Colorado police scratching their heads in puzzlement.

According to a report in the Denver Post, at least 17 fixed-wing drones with a six-foot wing span, regularly fly what appear to be search or mapping missions.

Phillips County Sheriff Thomas Elliot told the Post the drones have been sighted by residents for about a week flying between 7-10 p.m. The pilots’ identity remain a mystery and the FAA, DEA and Air Force say they don’t know the drones’ origins.

Elliot estimates the drones fly between 200-300 feet over a 25 square-mile area at speed of between 25-40 mph.

“They’ve been doing a grid search, a grid pattern,” Elliot said. “They fly one square and then they fly another square.”

Given its size and number, the swarm probably doesn’t belong to hobbyists.

The Post reports:

“On Friday, [Undersheriff William Myers] said he watched eight of the large drones flying along the Yuma County border near the intersection of U.S. 385 and County Road 54. At the same time, a single drone hovered about 25 miles away over the town of Paoli — it didn’t move all night, just hovered over the town — and eight more drones flew over Haxtun, about 10 miles down the road from Paoli – sometimes they fly over towns, other times over empty fields.”

Elliot suspects a private company may be responsible. The pattern is suggestive of a mapping or inspection flight profile.

“They do not seem to be malicious,” Elliott said. “They don’t seem to be doing anything that would indicate criminal activity.”

Drone photographer and educator Vic Moss of Drone U told the Post:

“We have a number of drone companies here in Colorado, and they’re very innovative. So maybe they’re testing something of theirs out in that area because it is very rural. But everyone that I know of, they coordinate all that stuff with local authorities to prevent this very situation. They all very much want people to understand drones and not cause this kind of hysteria.”

Since the drones likely weigh more than 55 pounds, the pilots are either FAA certified and have commercial waivers, or they are flying illegally.