One person died and seven US Marines and a Navy sailor were missing on Friday, a day after their amphibious assault vehicle (AAV) sank off the southern California coast during a training mission, Marine Corps officials said.
Seven other Marines were rescued and are alive while one Marine was killed after their vehicle took on water and sank around 5.45pm Pacific time on Thursday (0045 GMT Friday), US military officials said during a news conference.
"They signalled to the rest of the unit that they were in fact taking on water," Lt. Gen. Joseph Osterman said.
"Immediate response was provided by two additional amphibious assault vehicles as well as a safety boat."
Two of the rescued Marines were in a critical condition at Scripps Memorial Hospital La Jolla, while the other five are back on their assigned ships, Gen. David Berger said.
A search and rescue mission involving a US Navy destroyer and a Coast Guard cutter continued on Friday afternoon for the missing Marines and sailor.
The Marines were wearing combat gear along with inflatable vests when the incident occurred, Lt. Gen. Osterman said.
"It sank completely," he said, adding that it was in several hundred feet of water.
At "26 tonnes, the assumption is that it went all the way to the bottom".
The vehicle that sank is under hundreds of feet of water, putting it beyond the reach of divers and complicating rescue efforts for the missing troops, officials said.
The incident occurred during what the Marine Corps said was a routine training exercise near San Clemente Island.
Marines often practice beach assaults there using amphibious troop transport vehicles.
Gen. Berger said he suspended all AAV water operations until the cause was determined. He said AAVs across the fleet would be inspected.
All the Marines on the vehicle, which resembles a seafaring tank, were assigned to the 15th Marine Expeditionary Unit, which is based at Camp Pendleton, the largest Marine base on the West Coast of the United States, between Orange and San Diego counties.
Military ships, small boats and helicopters continued searching the choppy seas for the missing on Friday amid moderate to strong winds. The Navy-owned island is about 70 miles offshore from San Diego.
The Navy and Coast Guard were discussing ways to reach the sunken vehicle to get a view inside it, Lt. Gen. Osterman said.
All of the Marines on the vehicle ranged in age from 19 to early 30s and all were wearing combat gear, including body armour. Each troop also had flotation devices.
The vehicle is designed to hold up to 24 people with 280 pounds of equipment each, Lt. Gen. Osterman said.
He said there were three water-tight hatches and two large troop hatches and that it was designed to be naturally buoyant.
The accident marks the third time in recent years that Camp Pendleton Marines have been injured or died in amphibious assault vehicles during training exercises.
The vehicles have been used since 1972, and continually refurbished.
Marine Corps officials said on Friday that they did not know the age or other details of the one that sank.
In 2017, 14 Marines and one Navy sailor were hospitalised after their vehicle hit a natural gas line, igniting a fire that engulfed the landing craft during a training exercise at Camp Pendleton.
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 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 armoured 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 war-fighting organisation. There are three such groups which are made up of ground, air and logistics forces.