For the second time in 15 months, an armed sailor aboard a submarine undergoing maintenance at Pearl Harbor Naval Shipyard has died from an apparent self-inflicted gunshot wound.

The Navy said Tuesday that the sailor died Monday while on duty aboard the unidentified Pearl Harbor sub.

The Naval Criminal Investigative Service is investigating. The sailor’s identify and the submarine involved are being withheld pending notification of next of kin.

“We are deeply saddened by the loss of a shipmate. Our thoughts and prayers are with the sailor’s family, friends and shipmates,” the U.S. Pacific Fleet Submarine Force said in a release.

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 Pearl Harbor Naval Shipyard and then turned a pistol on himself while the sub was in Dry Dock 2 for a major overhaul, according to authorities.

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.

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

Navy investigators failed to uncover the reason for Romero’s actions, but a Navy report into the shootings found that mental health issues were under-diagnosed and were not adequately discussed with his chain of command — two shortcomings that might have allowed him to get proper treatment before his deadly rampage.

A redacted, 190-page report based on the work of a team of 22 investigators and experts, also said an apparent reluctance to diagnose Romero with disorders that would have disqualified him from duty may have been influenced by concerns to ensure adequate military staffing and readiness.

The Navy report gave no indication why the victims were targeted. An account of the incident seems to suggest it was just tragic timing. The three civilians had exited a work trailer alongside the submarine at around the same time Romero started his first patrol. They were gunned down near the bow of the submarine after they walked past him.

Although Romero’s motives remain a mystery, and he had no history of violence and apparently had not expressed any thoughts of harming himself or others, investigators said “the evidence showed he had several stressors in his life in the months leading up to the shooting that, when taken together, likely led him to choose violence.”

Those stressors included having been written up on numerous occasions for poor job performance, failing to pass a test for a promotion or meet other qualifications for advancement, social awkwardness and a sense of alienation from his shipmates.

Investigators compiled a detailed narrative of Romero’s contacts with military medical and mental health providers and of his lengthy record of disciplinary warnings and performance failings.

“This tragic event was heartbreaking to our community and our valued shipyard workforce, and we must work hard to restore confidence in the Navy’s ability to protect our most valuable assets – our people,” Adm. John C. Aquilino, commander of U.S. Pacific Fleet, said when the investigation results were released.

The Navy and Marine Corps conducted a broad review of security measures in the face of insider threats following three violent incidents at Navy bases, including the Pearl Harbor Naval Shipyard fatal shootings.

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Star-Advertiser writer Christie Wilson contributed to this report.