Fifteen Marines were wounded, six of them critically, when an amphibious vehicle they were training in caught fire at Camp Pendleton on Wednesday morning. The accident happened just after 9:30 a.m. at the base in Southern California, the Marine Corps said in a statement. The Marines, members of the First Marine Division, "were conducting scheduled battalion training" when the vehicle caught fire, the statement said. The cause is under investigation. Six of the Marines were listed in critical condition and six in serious condition. One other was hospitalized in stable condition, and two were treated on base for minor injuries. The accident involved an Assault Amphibious Vehicle, or A.A.V. These vehicles, which Marines use to move from sea to land, have been in use since the 1970s, the Marine Corps said in a second statement. First Lt. Paul Gainey, the First Marine Division's public affairs officer, said in an email Wednesday evening that he had no further information. Wednesday's episode was the third major accident involving Marines this summer. In July, a Marine Corps transport plane crashed in Mississippi, killing 15 Marines and a Navy corpsman. And in August, three Marines died when their Osprey aircraft crashed off the eastern coast of Australia. Twenty-three other military members on the plane were rescued. The Marine Corps briefly grounded its planes after the second crash in order to review safety measures.