The submarine tender USS Frank Cable arrives at Sasebo Naval Base, Japan, Oct. 14, 2024. (Karri Langerman/U.S. Navy)

Two Japanese contractors died last year after inhaling hydrogen sulfide gas while servicing a U.S. Navy vessel at Sasebo Naval Base, according to a Japanese government investigation.

The men — identified by police as Takahiro Sasahara, 54, and Seitaro Hamada, 33 — were found unconscious April 9, 2024, aboard a commercial barge used to purify wastewater from the ship, the Navy said at the time. Both were pronounced dead at local hospitals later that day, police said.

An investigation report released Thursday by the Japan Transport Safety Board said the men suffered cardiac arrest after entering a tank on the barge, the SEAON No. 3, that had not been cleaned for several months.

Sulfate-reducing bacteria in sludge at the bottom of the tank produced hydrogen sulfide, which displaced oxygen and created a toxic environment, according to the investigation.

“It is believed that Hamada stepped on the sludge when entering the tank, causing a release of hydrogen sulfide and further increasing the gas concentration inside the tank,” the report states.

Hamada entered the 16-foot-deep tank around 10:04 a.m. to retrieve a fallen filter bag, followed a minute later by Sasahara. Neither man measured gas levels or obtained authorization before entering, the report said, noting the operations chief was off that day.

A ship captain, unnamed in the report, discovered the men about an hour later, noticing an open hatch and shoes left nearby. U.S. firefighters recovered Sasahara at 12:43 p.m. and Hamada at 12:51 p.m., the report said. Japanese firefighters took them to separate hospitals, where they were declared dead about 90 minutes later.

Base spokesman Aki Nichols said by email Friday that the Navy cooperated fully with investigators and separately reviewed the incident “in the interest of identifying possible measures within our power to prevent a reoccurrence.”