The Marine Corps on Monday mourned the loss of eight Marines and a sailor who died in a mishap in which their floating armored vehicle took on water and sunk on Thursday evening off the coast of California in the deadliest incident at sea for the service in years.

The deaths were marked after the service shifted from search-and-rescue operations to a recovery mission off San Clemente Island, a Navy-owned training area. The dead were traveling in a 26-ton amphibious assault vehicle, and included nine men who are members of the 15th Marine Expeditionary Unit, of Camp Pendleton, Calif. Each of the service members was 23 or younger.

“I know all of us in the USMC family are extremely saddened following the announcement of the end of [search-and-rescue] operations,” tweeted Gen. David H. Berger, the commandant of the Marine Corps. “This difficult decision was made after all resources were exhausted. Our prayers continue to be with the family and friends of the 8 Marines and one Sailor we lost.”

The Marine Corps said in a statement that the service members presumed dead are Pfc. Bryan J. Baltierra, 18, of Corona, Calif.; Lance Cpl. Marco A. Barranco, 21, of Montebello, Calif.; Pfc. Evan A. Bath, 19, of Oak Creek, Wis.; Navy Hospitalman Christopher Gnem, 22, of Stockton, Calif.; Pfc. Jack Ryan Ostrovsky, 21, of Bend, Ore.; Cpl. Wesley A. Rodd, 23, of Harris, Tex.; Lance Cpl. Chase D. Sweetwood, 19, of Portland, Ore.; Cpl. Cesar A. Villanueva, 21, of Riverside, Calif.

Another Marine, Lance Cpl. Guillermo S. Perez, 20, of New Braunfels, Tex., was recovered last week and pronounced dead.

Defense Secretary Mark T. Esper offered condolences in a statement on Monday.

“Their service, commitment and courage will always be remembered by the nation they served,” Esper said. “While the incident remains under investigation, I want to assure our service members and their families that we are committed to gathering all the facts, understanding exactly how this incident occurred, and preventing similar tragedies in the future.”

Seven other Marines were recovered alive. Five have returned to their unit, and two remained hospitalized. One is in critical condition, and the other has since been upgraded to stable condition, the Marine Corps said.

The incident marks the deadliest at sea for the Marine Corps since 12 service members died when two CH-53E helicopter collided off the coast of Hawaii on Jan. 14, 2016, spawning a multiday search for survivors.

The vehicle, designed to carry infantrymen both at sea and on shore, has been in use since the 1970s. Lt. Gen. Joseph Osterman, the commanding general of I Marine Expeditionary Force, said in a news conference on Friday that the Marines had just completed a training exercise on San Clemente Island and were returning to their ship when the trouble began.

The service believes the vehicle is under a few hundred feet of water, complicating recovery efforts.

Marines have said over the years that the vehicles have become increasingly difficult to use and maintain, but they remained after the service attempted to field a replacement, the expeditionary fighting vehicle, but scrapped it in 2011 after $3 billion in development because of testing failures and rising costs. Another AAV sank off the coast of California in 2011, killing one Marine.

The service is now attempting to field another AAV replacement known as the amphibious combat vehicle.