The fire that engulfed a Marine armored vehicle Wednesday, injuring 14 Marines and one sailor during a training exercise in California, appears to have been caused by an exposed gas line, a U.S. official with direct knowledge of the investigation said Friday.

The Marine Amphibious Assault Vehicle, or AAV, was carrying a squad of infantrymen and had pulled off the road in what is known as a “herringbone” formation during a combat qualification exercise at Marine Corps Base Camp Pendleton, according to the official, who spoke on the condition of anonymity because the investigation is ongoing. During the maneuver, the roughly 30-ton vehicle appeared to have run over a gas line, severing it with its treads, and the line then somehow ignited. Fox News first reported that a ruptured gas line may have triggered the fire.

The ensuing explosion rapidly engulfed the vehicle in flames, and while no one was killed, some of the Marines, from both 1st Battalion, 1st Marines and the 3rd Assault Amphibian Battalion, were severely burned, including a staff sergeant who had previously been shot in the leg in Afghanistan but stayed in the service. The Marines have since been rushed to burn centers and medical facilities in the surrounding areas, the Marine Corps said in a statement Thursday.

It is unclear whether the vehicle’s fire suppression system went off or was overwhelmed. In service since the early 1970s, Marine AAVs have been continuously updated and upgraded. In 2014, the branch solicited a contract to outfit new fire suppression on the vehicles, according to online documents. In the past, the system was known to be temperamental, occasionally going off in Iraq just because of the ambient heat. In 2015, 22 Marines were hospitalized when the system accidentally went off, filling the cabin with halon and causing respiratory injuries.

The last major incident involving an AAV was in 2013, when a Marine was killed by the accidental detonation of a mine clearance charge stored aboard his vehicle. In 2011, a Marine also drowned when his vehicle sank off the California coast during a training exercise. Known by Marines as “tracs” or “yat-yas,” the vehicles are primarily used to transport troops from ship to shore and are lightly armored. They were, however, used during the invasion of Iraq and saw extensive combat near the city of Nasiriyah in 2003, and in the battle for Haditha in 2005. During the fighting, an AAV packed with Marines hit a massive roadside bomb, killing 15 of the 16 occupants.