(CNN) Mysterious drones have been flying over Colorado and Nebraska in recent weeks and authorities can't figure out who's behind the aircraft.

Deputies have spotted more than 16 unmanned drones flying in northeast Colorado after authorities received multiple reports of drone sightings last month, the sheriff's offices in Colorado's Yuma and Phillips counties said.

FAA spokesman Ian Gregor told CNN that multiple FAA divisions and government agencies are investigating reports of drones in Colorado and Nebraska.

Yuma County Sheriff Todd Combs said in a Facebook post Tuesday it appears the drones were staying at least 150-200 feet away from buildings and people and are flying in airspace controlled by the federal government.

The aircraft, Combs said, are operating within federal guidelines but are making residents "very nervous and anxious."

Read More