According to the Denver Post, a swarm of drones numbering anywhere from 17 to 30 have been appearing in the night sky above Colorado and Nebraska.

The drones are roughly six feet across, and have been appearing and disappearing at the same time every night, and stay between 200 and 300 feet off the ground. Based on a statement by Phillips County Sheriff Thomas Elliot, the drones appear to be searching for something or mapping the terrain.

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

The Federal Aviation Administration (FAA) said it did not know where the drones came from or who was operating them. Meanwhile, the Air Force, US Army Forces Command and Drug Enforcement Administration all denied the drones were theirs. Given the size and numbers of drones, it seems likely the drones are being flown by a company or agency, as an operation of this size would likely be prohibitive for hobbyists.

The FAA recently proposed a new rule that would give it the authority to identify and track the majority of drones in the skies. Such a rule would make it much easier for the FAA to know who the drones belong to and exactly what they’re doing.