They were traveling in the vehicle that resembles a seafaring tank from the shores of San Clemente Island to a Navy ship Thursday evening when they reported the vehicle was taking on water, said Lt. Cameron H. Edinburgh, a Marine Corps spokesman for Camp Pendleton.
Two Marines who were rescued were injured, with one hospitalized in critical condition and the other in stable condition, a Marine Corps statement said.
Military ships, small boats and helicopters on Friday were searching choppy seas for the missing amid moderate to strong winds. The Navy-owned island is about 70 miles offshore from San Diego.
All of the Marines on the vehicle were assigned to the 15th Marine Expeditionary Unit and were involved in a routine military exercise when the vehicle started taking on water, the Marine Corps said.
"We are deeply saddened by this tragic incident. I ask that you keep our Marines, Sailors, and their families in your prayers as we continue our search," Col. Christopher Bronzi, the unit's commanding officer, said in a statement from the Marine Corps.
“We are deeply saddened by this tragic incident. I ask that you keep our Marines, Sailors, and their families in your prayers as we continue our search,” said Col. Christopher Bronzi, 15th MEU Commanding Officer.— I MEF (@1stMEF) July 31, 2020
Thursday's accident marks the third time in less than a decade that Camp Pendleton Marines have been injured or died in amphibious assault vehicles during training exercises.
In 2017, 14 Marines and one Navy sailor were hospitalized after their amphibious assault vehicle hit a natural gas line, igniting a fire that engulfed the landing craft during a training exercise at Camp Pendleton, the sprawling coastal Marine Corps base north of San Diego.
And in 2011, a Marine died when an amphibious assault vehicle in a training exercise sank off the shores of Camp Pendleton.
The Marines use the amphibious assaults vehicles to transport troops and their equipment from Navy ships to land. They are nicknamed "amtracs" because the original name for the vehicle was "amphibious tractor."
The armored vehicles outfitted with machine guns and grenade launchers look like tanks as they roll ashore for beach attacks, with Marines pouring out of them to take up positions.
The Marine Expeditionary Force is the Marine Corps' main warfighting organization. There are three such groups which are made up of ground, air and logistics forces.