Two sewer maintenance workers fell unconscious and two others suffered minor injuries Friday at a work site in Yokosuka, Kanagawa Prefecture, in what police and firefighters suspect was a case of gas poisoning.

Three of the four workers were briefly in a state of cardiac arrest when found near a manhole by firefighters responding to an emergency call around 9:15 a.m. Hydrogen sulfide was detected at the scene.

The city said six men from a private company began work at around 9 a.m., replacing parts in equipment that directs sewage to a treatment facility.

Workers are supposed to confirm the absence of hydrogen sulfide before entering manholes.

The accident took place in a residential area with a school and a shopping center nearby. It is about 400 meters south of JR Kurihama Station.

Residents said they saw a large amount of white foam coming out of the manhole.