SpaceX‘s massive Starship spacecraft exploded into a dramatic fireball during testing in Texas late on Wednesday, the latest in a series of setbacks for billionaire Elon Musk’s Mars rocket program.

The explosion occurred around 11 p.m. local time while Starship was on a test stand at its Brownsville, Texas Starbase while preparing for the tenth test flight, SpaceX said in a post on Musk’s social-media platform X.

The company attributed it to a “major anomaly,” and said all personnel were safe.

“Preliminary data suggests that a nitrogen COPV in the payload bay failed below its proof pressure,” Musk said in a post on X, in a reference to a nitrogen gas storage unit known as a Composite Overwrapped Pressure Vessel. “If further investigation confirms that this is what happened, it is the first time ever for this design,” he continued.

SpaceX didn’t immediately respond to a request for further comment.

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(Reporting by Jaspreet Singh in Bengaluru and Cassel Bryan-Low in London; Editing by Joe Brock and Aidan Lewis)