A crewman who stabbed his supervisor to death aboard a container ship heading to Los Angeles was sentenced to 20 years in federal prison on Monday.

Michael Dequito Monegro, 44, of the Philippines, was sentenced in federal court in Los Angeles. He pleaded guilty in May to committing an act of violence against someone aboard a ship that is likely to endanger the vessel’s safe navigation.

Monegro was working on the MSC Ravenna on a two-week run from Shanghai to Los Angeles in September 2020 when he stabbed the man as the vessel was about 92 miles (150 kilometers) off the Southern California coast, according to his plea agreement.

Monegro attacked his supervisor in a hallway outside a locker room, prosecutors said.

The two men struggled and fell down. Monegro got on top of his victim, stabbed him, pulled a second knife from his supervisor’s coveralls and attacked him with both knives despite other crewmembers trying to stop him, including throwing a trash can at him, prosecutors said.

Monegro stabbed the victim 31 times, according to a statement Monday from the U.S. attorney’s office.

“Monegro stopped stabbing the victim only when he became too tired to continue,” the statement said.

The ship’s captain, chief mate and chief engineer arrived and the captain finally persuaded Monegro to get off the victim, who died on the ship, the statement said.

Monegro was kept under guard in his cabin and arrested a week later when the ship docked at the Port of Los Angeles.

Authorities didn’t indicate a motive for the attack. The victim, identified in court papers only by the initials “M.S.,” left a wife and a daughter who was 17 at the time. He was the only income earner “and his death caused significant financial strain on the family,” prosecutors said in a sentencing memorandum.