Nine service members drowned when their vehicle sank off of California.

The U.S. Marine Corps has fired the commander in charge of the unit involved in a deadly training accident off the coast of Southern California in July that killed eight Marines and a Navy sailor. The service members drowned after their amphibious assault vehicle sank during training on San Clemente Island.

“Lt. Gen. Karsten S. Heckl, Commanding General, I Marine Expeditionary Force, relieved Lt. Col. Michael J. Regner, Commanding Officer, Battalion Landing Team, 1st Battalion, 4th Marine Regiment, 15th Marine Expeditionary Unit today,” said a Marine Corps statement Tuesday.

“Heckl relieved Regner due to a loss in trust and confidence in his ability to command as a result of the assault amphibious vehicle mishap that took place off the coast of Southern California July 30, 2020,” it added.

The Marine statement said that the ongoing investigation incident had “compiled a substantial amount of information and data which formed the basis for Heckl’s decision.”

The battalion landing team, a reinforced infantry battalion, is the ground combat element of a Marine Expeditionary Unit, a force of 2,200 Marines that sail aboard three Navy amphibious ships in rotating deployments.

On July 30, 15 Marines and a sailor were aboard the AAV as it made its way back to the U.S. Navy amphibious ship USS Somerset after having trained on San Clemente Island, a Navy-owned island off the coast of southern California that is used by for training.

Some of the personnel aboard were able to be rescued from the sinking craft, including a Marine who later died ashore from his injuries.

But seven other Marines and a Sailor remained trapped inside and drowned as the AAV quickly sank hundreds of feet to the ocean floor. A salvage effort recovered their remains a week later.

Ranging in age from 18 to 22, the service members killed in the incident were Pfc. Bryan J. Baltierra, Pfc. Evan A. Bath, Pfc. Jack-Ryan Ostrovsky, Lance Cpl. Marco A. Barranco, Lance Cpl. Guillermo S. Perez, Lance Cpl. Chase D. Sweetwood, Cpl. Wesley A. Rodd, Cpl. Cesar A. Villanueva and Navy Hospital Corpsman 3rd Class Christopher Gnem.

There have been 10 to 15 reported incidents over the past 20 years involving AAVs, with the most recent report involving a water-based fatality happening in January 2011.

“San Clemente is a very challenging amphibious training ground,” said Eric Oehlerich, an ABC News contributor and former Navy SEAL who’s conducted trainings there. “Night amphibious training is some of the most complex and high-risk training you can do as an amphibious soldier.”

The 15th MEU continues “to train in preparation for crisis and contingency response,” according to the Marine Corps.