What are the mystery drones flying over the US?

Mysterious drones have been swarming the night skies above New Jersey and other nearby states for a month. They've been spotted over several US military sites. They've been videoed over houses and apartment buildings. A swarm was seen following a US Coast Guard rescue boat at the same time that New Jersey police reported 50 drones arriving on land from the ocean. But no one seems to know who's piloting them, or whether it's a coordinated effort.