Marines with amphibious assault vehicles during an exercise at Camp Pendleton, Calif., in 2014. (Sgt. Melissa Wenger/Marine Corps)

Fifteen Marines were hurt, some severely, when their armored amphibious vehicle caught on fire Wednesday during a training exercise in southern California.

The incident occurred at Marine Corps Base Camp Pendleton, approximately 40 miles north of San Diego, about 9:30 a.m. local time, military officials said. Everyone involved was hospitalized, though the severity of their injuries varied substantially.

Eight Marines were rushed to the burn unit at University of California San Diego Health, a local medical center. Three are listed in critical condition and five are in serious condition, military officials said.

Four others were taken to the University of California Irvine Medical Center, where two are in critical condition and two are receiving care but their status was not immediately known, officials said.

One Marine is in stable condition at Scripps Memorial Hospital in La Jolla, Calif.

Two others are being treated for minor injuries at Camp Pendleton’s naval hospital.

It’s unclear how the blaze began or how it was extinguished. In a written statement, military officials said the matter is being investigated.

They’ve said little else.

Those injured are members of 1st Battalion, 1st Marines, an infantry unit, and 3rd Assault Amphibian Battalion, which operates the amphibious tractors. Both are based at Camp Pendleton.

Used to carry combat troops ashore from larger naval vessels at sea, amphibious assault vehicles — Marines call them amtracs, or simply AAVs — were first fielded in the 1970s. The Marine Corps is in the midst of a long, expensive effort to replace its AAV fleet.

As Military.com’s Hope Hodge Seck reported Wednesday, the last major accident involving an AAV occurred four years ago when ordnance on the vehicle ignited during a training event, killing one Marine and injuring four others. As a result, the Marines stopped using the vehicles’ older mine clearance weapons, introducing a safer variant in August.

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