Fifteen U.S. Marines were injured during a training exercise Wednesday morning when the amphibious vehicle they were using caught fire at California’s Camp Pendleton, The New York Times reported. Six of the injured Marines are critically wounded and six others are in serious condition. Another is in stable condition after being hospitalized, and two others received only minor injuries. The cause of the fire is under investigation. The accident involved an assault amphibious vehicle, which Marines have used since the 1970s to move from sea to land, the Marine Corps said in a statement. Wednesday’s incident is the third major Marines accident this summer. A Marine Corps transport plane killed 15 Marines and a Navy corpsman when it crashed in Mississippi in July, and three other Marines died in an aircraft crash in August.