A mysterious fleet of drones entered restricted airspace and swarmed a U.S. military base along the Virginia coast for 17 days late last year, stumping the Pentagon, according to a new report. 

For several nights last December, U.S. military personnel reported witnessing a fleet of unknown unmanned aircraft breach restricted airspace over a stretch of land at Langley Air Force Base along Virginia's shore, the Wall Street Journal first reported. 

The drones would start to arrive about 45 minutes to an hour after sunset each night, one official reportedly told U.S. Air Force Gen. Mark Kelly, who joined several other officers responsible for the country's most advanced jet fighters, including F-22 Raptors, on a squadron rooftop. 

Kelly described the first drone he saw as roughly 20 feet long and flying at more than 100 miles an hour, at an altitude of roughly 3,000 to 4,000 feet. As many as a dozen or more drones followed, flying across Chesapeake Bay, and then traveling toward Norfolk, Virginia, and through a space overlooking the base for the Navy’s SEAL Team Six and Naval Station Norfolk, the world’s largest naval port, according to the Journal.