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.

A Facebook FB, -1.20% spokeswoman declined to share details about the damage or cause of the failure ahead of the NTSB’s report. A spokeswoman said Facebook viewed the Aquila test flight a success. “We have already learned a lot from the results of this flight test and will continue to learn from all the future flight tests we plan to run,” she said.

The NTSB, whose primary function is to probe accidents, plans to release a more detailed report on the accident within the next two months. Bloomberg first reported on the accident Monday.

An expanded version of this report appears on WSJ.com.

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