SAN DIEGO — The coronavirus pandemic that curtailed trainings in 2020 contributed to nine service members drowning off San Diego's coast, according to a new military investigation into one of the Marine Corps' deadliest training accidents in recent years.

Senior commanders leading up to the accident also were strapped with extra "nonstandard" missions including sending Marines to the U.S.-Mexico border as part of the Trump administration's tightening of border security and assisting with the Navy's hospital ship, the USNS Mercy, that anchored off Los Angeles to relieve hospitals overwhelmed with coronavirus cases, according to investigation findings made public Wednesday.

The amphibious assault vehicle sank on July 30, 2020, off San Clemente Island, trapping troops inside it. A previous investigation found the deaths were preventable and blamed the tragedy on inadequate training, shabby maintenance of the 35-year-old amphibious assault vehicles and poor judgment by commanders.

The families of the eight Marines and one sailor have filed a lawsuit against BAE Systems, the manufacturer of the amphibious assault vehicles, alleging the company knew for a decade or more about a design defect that makes it nearly impossible for troops to open the cargo hatches and escape the 26-ton amphibious vehicles when they sink.

The vehicles have been at the heart of the Marine Corps' amphibious operations, carrying troops from ship to shore for both combat and humanitarian operations since the early 1980s. The armored 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 findings released Wednesday looked at the readiness of the troops before they participated in the exercise 70 miles (113 kilometers) off San Diego's coast and noted that it should not take away from the the earlier probe that found a slew of missteps and oversights that left the crew in the dark and using their cell phone lights to desperately try to find an unmarked escape hatch as they took on water. There were also no safety boats nearby to save them.

Even so, Lt. Gen. Carl Mundy III wrote that "it would be a mistake to discount or overlook the extraordinary COVID-related demands on leaders, staff, and their Marines and Sailors during this period."

He called the pace of keeping up with ever-evolving guidance on curbing the spread of the virus "immense."

"The claims on their time and attention surfaced in a number of interviews with several senior officers who described the conditions during this period as second only to their experience in combat," Mundy wrote.

About a dozen Marine officers have been forced out of their jobs or disciplined in another way. The Marine Corps also relieved a two-star general who had overseen the exercise.

A parallel Navy probe found there were communication problems between the branches.

The military has taken steps to prevent another tragedy, including requiring safety boats to be nearby.

The investigation found the troops had not received appropriate instruction on how to escape a sinking amphibious vehicle and that the unit had not completed a required evaluation meant to address any issues, including their swimming qualifications.

"Amphibious operations are inherently complex and dangerous, which places a premium on proper training and equally constant efforts to monitor and mitigate risks," Mundy wrote.