WASHINGTON (Dispatches) -- The U.S. military said Sunday it has called off a search for seven Marines and a sailor missing at sea for days, saying they were presumed dead.
The military personnel were aboard an amphibious assault vehicle (AAV) that sank Thursday in deep water off the coast of California during a training exercise.
"It is with a heavy heart that I decided to conclude the search and rescue effort,” said Colonel Christopher Bronzi, commander of the 15th Marine Expeditionary Unit.
During the 40-hour search, Marine, Navy and Coast Guard helicopters, ships, and watercraft searched more than 1,000 square nautical miles, the Marines said in a statement.
A total of 16 service members were on the amphibious vehicle — until now eight were rescued but one later died, and two others are in critical condition.
The search and rescue operation now shifts to one aimed at finding the bodies of the missing service members, Bronzi said.
The vehicle was on its way back to the U.S. Navy amphibious ship USS Somerset when it sank, according to three defense officials.
It was among a group of 13 AAVs returning to the ship, which was approximately a mile from shore, Lt. General Joseph Osterman, commander of I Marine Expeditionary Force, said at a press conference.
Osterman said that the personnel aboard the ill-fated vehicle signaled to other AAVs that they were taking on water. "It sank completely,” he said, adding that "the assumption is it went all the way to the bottom,” several hundred feet below the surface, too deep for divers.
One of the rescued Marines died later, and two others were rushed to local hospitals and last reported in critical condition.
AAV training on land would continue although operations on water were being suspended "out of an abundance of caution,” Osterman said.
AAVs are used to carry out beach landings. The small, armored craft are launched from Navy amphibious ships and convert into armored personnel carriers on land.
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 has conducted trainings there. "Night amphibious training is some of the most complex and high-risk training you can do as an amphibious soldier.”