Authorities say an investigation into reports of large drones flying in groups after dark since November has confirmed nothing unusual or criminal.

They're mostly hobbyist drones, commercial aircraft, stars, planets and weather phenomena.

Yet if history is any guide, the investigation involving flights of a heat-detecting plane in northeastern Colorado will settle nothing for many people.

They've also been seen in Nebraska and Wyoming.

Dan Carlson says drones have flown after dark near his Nebraska farm four times this year. He suspects military involvement.

U.S. Air Force officials overseeing dozens of nuclear missiles scattered across the region deny any role in the drone sightings.

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