Mysterious drone swarms flying at night are baffling US authorities

Mysterious drone swarms have been seen flying in Colorado, Nebraska and Wyoming at night since December, sometimes over locations believed to house nuclear missile silos. A federal task force has been formed to investigate the drones' origin and purpose. The Phillips County Sheriff's Office in Colorado reported the first drones on 20 December. There have been hundreds of sightings since, some of groups of drones flying in grid patterns. Some observers assumed the drones were part of a military exercise, but the US Air Force has denied involvement.