Facebook's project which involves the use of unmanned solar-powered drones to deliver Internet access has hit a major roadblock. Facebook's Aquila drone is currently under investigation by the United States National Transportation Safety Board following an incident that occurred during the first flight test of the drone.

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The test flight took place in July, and following the incident, Facebook reported that the drone suffered a "structural failure." Facebook did not elaborate on what happened to the drone. However, with a federal agency now investigating the matter, some suspect that it might be a major issue.

People with insider knowledge about the incident say that it happened just about 20 feet about the ground when the drone was making its final descent. The crew testing the drone tried to fly the aircraft in weather conditions that did not conform with the original plan, according to The Guardian.

In an emailed statement acquired by Bloomberg, Facebook said, "We are happy with the successful first test flight and were able to verify several performance models and components including aerodynamics, batteries, control systems and crew training, with no major unexpected results."

Facebook said that the Aquila was able to stay airborne for 96 minutes, which is three times the expected duration. Facebook plans to reach altitudes above 60,000 feet on its next test flight.

Facebook's Aquila drones have a 141-meter wingspan, which makes it bigger than a Boeing 737. Powered by electric motors which are charged by solar panels, the drones are designed to stay airborne for extended periods of time. The aim is to beam down wireless Internet connections into some of the most remote areas in the world.

Aquila was developed by a dedicated Facebook engineering team based in Bridgwater, Somerset. The firm working on the project was formerly known as Ascenta, and was acquired by Facebook in $2014 for $20 million.