A series of mysterious drones have been spotted flying over several counties in northeastern Colorado.

Sheriff Tom Nestor of Lincoln County told ABC News there have been two confirmed reports of drone sightings in his county over open areas such as ranches. Residents first noticed the drones on Dec. 17.

Residents are starting to speculate why the drones are only being operated at night.

Nestor said he’s concerned the local airport’s emergency helicopter could collide with one of the drones.

Rick Breitenfeldt of the Federal Aviation Administration (FAA) told ABC News, “This area is a pretty isolated, rural area and not a controlled air space.”

“As soon as the Governor heard about the sightings, he asked Department of Public Safety to find out what was going on. He’s actively monitoring reports of drone sightings in eastern Colorado and shares the expressed concerns of law enforcement and local residents. The Governor has not yet received any substantial information beyond what has been reported. While it remains a mystery about who is flying these drones and why, it’s certain they have caused a bit of a stir and the Governor wants to get to the bottom of it and make sure that no person or property is at risk,” a spokesperson from Colorado Governor Jared Polis’ office told ABC News.

Read more: yahoo