But ultimately the service concluded none of those shortfalls was directly to blame for the incident, which killed eight Marines and a Navy corpsman, according to a report released this week that was more than 800 pages long.
The probe was launched in April after the Marine Corps fired a colonel, a lieutenant colonel and then a two-star general for their roles in the incident. The AAV sank hundreds of feet beneath the surface on its way back to the amphibious transport dock Somerset on July 30, 2020.
The incident happened on the tail end of a day-long exercise, according to the report. The plan was for Marines embarked on the Somerset to launch a mechanized raid on San Clemente Island off the coast of San Diego.
However, the day ended up being fraught with issues.
Fourteen amphibious assault vehicles (AAVs) were supposed to launch, but one broke down inside the transport ship. Another broke down on the island. The AAV that would ultimately sink had issues itself, and Marines added six gallons of transmission fluid to the vehicle.
Later, nine AAVs were returning to the Somerset at around 4:45 p.m. that day when the ill-fated vehicle experienced mechanical failure and began taking on water.
As the vehicle began to take on more water, its commander ordered the Marines to prepare to get out. However, another AAV that came up to help accidentally bumped into the sinking vehicle due to choppy seas, and the impact turned it sideways into the waves.
With a hatch open, a wave quickly filled the troop compartment and the AAV sank with most of its crew still on board.
A subsequent investigation by the Marine Corps found that no single problem caused the AAV to sink that day. Instead, a "sequence of mechanical failures" caused the tragedy.
The Navy's investigation -- the fourth into what leaders have called a truly preventable tragedy -- was released Wednesday, and it doesn't differ significantly from the Marine Corps' findings. It offers few new details or information.
In fact, U.S. Pacific Fleet commander, Adm. Samuel Paparo, noted that "the Navy and the Marine Corps investigations established that poorly-maintained AAVs, inadequately trained personnel and the failure to conduct a timely egress caused the sinking of the AAV and the tragic loss of life" in his endorsement of the findings.
Vice Adm. Steve Koehler, commander of U.S. 3rd Fleet in San Diego, California, in his letter to Paparo, wrote that all the issues raised in the Navy's investigation "were not causal or contributory to the sinking."
Investigators did find, however, that the crew of the Somerset was unsure who had the authority to control the departure of the vehicles from the beach. Also, briefings leading up to the exercise "lacked commonly expected detail," and communication within the ship was "ineffective" throughout the exercise.
The communication issues were highlighted when another AAV with mechanical issues returning from the island caused confusion for the ship's crew.
"There were simultaneous discussions about which AAV was taking on water, how much water it had taken on, and which AAVs were trying to get back to [San Clemente Island] versus trying to return to the ship," investigators wrote.
Instead, the Navy's recommendations following this tragedy focus largely on training and doctrine. Paparo's letter said that the Navy will recommend that all incoming amphibious ship commanders and executive officers get additional amphibious warfare training.
Vice Adm. Roy Kitchener, commander of the Navy's surface forces in the Pacific, said in a statement that the service is "reworking procedures and doctrine, clarifying aspects of amphibious operations, and instituting new training requirements to prevent future tragedies."
The Navy is also making changes to its Wet Well Manual, a document that outlines the rules of launching and recovering craft on amphibious ships.
The service has clarified and tightened the rules around safety boats. Specifically, it will no longer allow another AAV to act as a safety boat -- something that occurred during the launch of the vehicles on July 30, 2020.
It is also requiring ships to have better control over the direction and status of landing craft and to better integrate Marine and Navy training.
Paparo said in his letter that, "unless otherwise directed," he plans to take administrative action in the cases of the commander of the amphibious task force, Capt. Stewart Bateshansky; Somerset's commanding officer, Capt. John Kurtz; and the ship's tactical action officer, who was not identified in the report.
When asked for more detail, Lt. Andrew Bertucci, a Navy spokesman, said that the service "did take administrative actions on some personnel involved" but that he was "not able to provide more information due to the privacy act."
In contrast, the Marine Corps relieved Lt. Col. Michael J. Regner, commanding officer of Battalion Landing Team, 1st Battalion, 4th Marines, in October 2020, and Col. Christopher Bronzi, the 15th Marine Expeditionary Unit's commanding officer in March 2021 over the incident.
In June of this year, the Corps also fired Maj. Gen. Robert Castellvi as the service's inspector general. Castellvi was the commanding general of the California-based 1st Marine Division at the time of the incident.
In all, 12 Marines have been or will be punished for their roles in the accident, Marine Corps spokesman Capt. Ryan Bruce confirmed to Military.com.
"The Navy and Marine Corps learned from this tragedy, and we are codifying the lessons we have learned as an organization so that the deaths of these Marines and sailor are not in vain," Kitchener said.
-- Konstantin Toropin can be reached at email@example.com. Follow him on Twitter @ktoropin.