People claim they have seen large groups of drones flying through the night sky across rural areas of the United States.Their mysterious appearance has raised questions and launched an investigation.Officials in the western state of Colorado say they have found nothing unusual or criminal about the drones.They were mostly operated for personal enjoyment, commercial purposes, or weather-related events.But the investigation that followed the drones in Colorado with sightingsinto Wyoming and Nebraska will not ease the minds of many people.Theories about drones said to be as big as cars, flying in organized groups at night continue to spread.Dan Carlson claims to have seen the drones four times after dark near his farm outside Sidney, Nebraska.He says the military or defense companies may be involved.The drones flew in groups of two on two nights, he said.Their speed, the distance they traveled, and the nearby missile sitesmakes him suspect the U.S. Air Force knows of the flights.Carlson believes the drones may be involved in some kind of military search-and-recovery practice.He suggests they are seeking out items hidden around the countrysideduring the daytime for training exercises at night."I do not buy into the conspiracy theories. I am not living in fear of an alien invasion," he told the Associated Press.F.E. Warren Air Force base in Cheyenne, Wyoming, oversees 150 nuclear missile sitesin the three neighboring states of Colorado, Wyoming and Nebraska.The missiles need regular checking, maintenance and protection against threats.But Air Force officials say they have not seen the drones and they have nothing to do with them.An Air Force spokesman said, "Our base is kind of a drone no-fly zone.So we do have ... training that goes on ... But any drones spotted outside ... are not part of our fleet."Colorado state officials reported that the state has over 24,000 registered drones.And there were over 2,200 drone sightings in Colorado in 2018.In early January, Colorado officials used a heat-detecting planeworking with investigators on the ground to follow reports of drone sightings.Of 23 drone reports, they found 13 small drones were flown for personal enjoyment,six were commercial aircraft, and four were unidentified.Carlson said the drones he saw came within 2 to 3 kilometers of his house,flying about 250 meters high. One sped away when he drove after it.On cold winter nights, Carlson said he can usually hear the sound of an airplane clearly."It was the kind of night that if an airplane flies over at 30,000 feet, you hear it. No sound with these," he said.