U.S. Marines with Bravo Company, Battalion Landing Team 1/4, 15th Marine Expeditionary Unit, operate assault amphibious vehicles into the well deck of the amphibious landing dock USS Somerset.
CNN  — 

At least one US Marine is dead and eight others are missing after a training accident involving an amphibious vehicle off the coast of Southern California.

Search and rescue efforts were still underway Friday night for the seven missing Marines and one sailor, with support from the Navy and Coast Guard.

At least two others were injured in the accident, which occurred Thursday, and have been transported to Southern California hospitals, according to a news release from the US Marine Corps 1st Expeditionary Force.

One is in critical condition and the other is stable, according to the Marines.

“Deeply saddened by, and closely monitoring, this tragic incident. Please keep our Marines, Sailors, and their families in your prayers as the search continues,” Secretary of Defense Mark Esper tweeted.

The incident occurred during a 15th Marine Expeditionary Unit and Makin Island Amphibious Ready Group routine training exercise in the vicinity of San Clemente Island, according to the Marines.

The incident began around 5:45 p.m. local time on Thursday, when Marines in an amphibious assault vehicle reported they were taking on water. Sixteen people were inside the vehicle at the time of the accident. Eight have been recovered.

One Marine died, two Marines are in critical but stable condition and five others are back aboard their ship, Commandant of the US Marine Corps Gen. David Berger said in a press briefing.

“I’ve directed an immediate suspension of amphibious assault vehicle water operations until the causal factors of this mishap are better understood,” Berger said. “First step is to conduct a search and rescue and take care of their families, so that’s what the focus is today.”

The amphibious assault vehicle weighs approximately 26 tons, and is believed to have sunk to the ocean floor, which is hundreds of feet deep in that location. The AAV is below the depths where divers can go, so the Navy is assisting and providing resources that can go down and take a look at the vessel, according to Marine Lt. Col. Jody Osterman.

The Marines who were found were wearing normal combat gear including body armor, Osterman said. That includes an inflatable vest. Some of the Marines who were picked up were floating in the ocean.

A Coast Guard cutter and helicopters from the Navy, Marines and Coast Guard are being used in the search, along with a destroyer and other surface vessels.

“Literally every asset we had that was available to search” was launched, Osterman said.

All of the service members in the incident are assigned to the 15th MEU based out of Camp Pendleton.

“We are deeply saddened by this tragic incident,” said Col. Christopher Bronzi, the commanding officer of the 15th MEU. “I ask that you keep our Marines, Sailors, and their families in your prayers as we continue our search.”

The name of the Marine killed will be withheld until 24 hours after next of kin have been notified. The incident is under investigation.

This story has been updated with additional information on the search Friday night.

CNN’s Shawn Nottingham contributed to this report.