A 23-year-old torpedoman’s mate 3rd class from California assigned to the submarine USS Charlotte was the sailor who died Monday from an apparent self-inflicted gunshot wound on a nighttime watch as the sub was pierside at Pearl Harbor Naval Shipyard, the Navy said Wednesday.

“Every member of our Navy team is important; this is a painful time for our local community and the U.S. Navy,” the U.S. Pacific Fleet Submarine Force said in a release Wednesday.

The Naval Criminal Investigative Service is investigating.

It was the second time in 15 months that an armed sailor aboard a submarine undergoing maintenance at the shipyard has died from an apparent self-inflicted gunshot wound.

The sailor in Monday’s shooting was an armed watch-stander. Submarine sailors are not allowed to carry personal weapons aboard submarines.

On Dec. 4, 2019, Machinist’s Mate Auxiliary Fireman Gabriel Antonio Romero of San Antonio, 22, an armed watch-stander on the attack submarine USS Columbia, shot three civilian workers, killing two, at the Pearl Harbor shipyard and then turned a pistol on himself while the sub was in Dry Dock 2 for a major overhaul.

Vincent Kapoi Jr., 30, and Roldan Agustin, 49, were killed and Roger Nakamine, 36, was injured in the shooting that resulted in a base lockdown and sent shockwaves through the state’s largest industrial employer, which has more than 6,400 mainly civilians workers.

Last April, following an investigation, Adm. John Aquilino, commander of U.S. Pacific Fleet at Pearl Harbor, wrote that Romero was solely responsible for the tragedy — and that the investigation could not find any motive for the shooting.

“After reviewing this investigation, I reached the conclusion that even if all of Romero’s mental health, personal issues and grievances had been known and taken into consideration, no one could have reasonably predicted that he would engage in this ultimate act of murder and suicide,” Aquilino said.

The failure to identify Romero as an “insider threat” led to doubts about the necessity of armed watch-stander requirements in “low force protection posture environments” and where layered security already exists, he said.

Submarines in port typically have multiple armed crew member watch-standers. Aquilino added the importance of protecting critical national security assets must not be understated.

“However, I am also acutely aware of how this tragedy shook the sense of safety and security of our Navy community, especially our valued shipyard workforce,” he said.

Aquilino made over 70 recommendations to improve safety.

Pearl Harbor Naval Shipyard & Intermediate Maintenance Facility undocked USS Charlotte, a Los Angeles-class fast-attack submarine, on Jan. 26 from Dry Dock 3, with the Navy calling it a “major milestone” in the submarine’s “engineered overhaul” maintenance availability.

The submarine, which has a crew of 130 to 150, was pierside in the shipyard Monday as it finalizes repairs.

An engineered overhaul is a major multi-year overhaul near the mid-point of a submarine’s service life to perform necessary repairs, maintenance and modernization to ensure the submarine is operating at full technical capacity and mission capability.

Charlotte has been in the shipyard since April 2019, the Navy said.