A U.S. air-safety watchdog is investigating an accident during the first flight of a Facebook Inc. drone designed to extend internet access.

The giant drone, called Aquila, experienced a “structural failure” while approaching Yuma, Ariz. during a test flight on June 28, according to a spokesman for the U.S. National Transportation Safety Board. There were no injuries, but the drone was “substantially damaged” and the damage “compromised the airworthiness of the aircraft,” the spokesman said.