Two Marine Corps maintainers were severely burned Sunday when a fire broke out aboard an F/A-18 Hornet at Marine Corps Air Station Miramar, California, according to the 3rd Marine Aircraft Wing.

“Both Marines are currently being treated for severe burns at the UC-San Diego Regional Burn Center,” the wing said in a statement. “Our thoughts and prayers are with them.”

One of the maintainers is in critical condition, said Maj. Kurt Stahl, a spokesman for the wing. They were assigned to Marine Fighter Attack Squadron 112, a Reserve unit based out of Fort Worth, Texas.

Media outlets are reporting that the maintainers were burned when the F/A-18 they were working on exploded, but the wing described the event as a “ground flash fire,” which technically is smaller than an explosion.

The damage to the aircraft is still being assessed, and investigators are working to determine what caused the fire, Stahl said on Monday.