Facebook's Aquila drone.
Bloomberg reports that the National Transportation Safety Board (NTSB), a US agency, is investigating after an incident during its first test flight of its "Aquila" drone that left it with "substantial damage."
The social networking company had previously mentioned - without elaborating - "a structural failure we experienced just before landing" in a July blog post about the test flight.
But the test was otherwise presented as a big win, with The Verge's Casey Newton - who was given access to the Facebook team after the test - writing that the company "presented the flight as an unqualified success."
However, the drone was reportedly "substantially damaged," meaning it is no longer "airworthy."
The Aquila drones are huge, with 43-meter wingspans - bigger than a Boeing 737. They're designed to stay aloft for months at a time at altitudes of 60,000, powered by solar energy and beaming down wireless internet around the world. It's an ambitious project - and it looks like there are going to be challenges along the way.
"We were happy with the successful first test flight," a Facebook spokesperson told Business Insider in a statement, "and were able to verify several performance models and components including aerodynamics, batteries, control systems and crew training, with no major unexpected results."
One source with knowledge of the incident claimed that it happened 20 feet above the ground on a final decent, and that it happened after the team flew the aircraft in conditions that were windier than planned.
It's not clear why Facebook did not disclose that the NTSB was investigating.