Four male workers in their fifties lost their lives in Gyoda City, Saitama Prefecture, after falling into a manhole during a sewer inspection, authorities reported on Sunday.
The incident took place on Saturday, during a routine inspection of underground sewage pipes.
According to local emergency services, one worker accidentally fell into a manhole over 10 meters deep.
Three other workers attempted to rescue him but also fell in.
All four were later pulled out and taken to a nearby hospital, where they were pronounced dead.
The workers were part of a team of at least seven individuals involved in a countrywide survey to assess the condition of sewage systems.
They had been instructed to inspect and clean the pipes if necessary.
Following the fall, rescue crews detected hydrogen sulfide gas at the scene.
This toxic substance can cause breathing problems or loss of consciousness when inhaled in large amounts.
Investigators believe the gas may have played a role in the accident, possibly affecting the workers’ ability to react or escape.
Local police and fire departments are still looking into the sequence of events that led to the deaths.
So far, no official statement has been made regarding responsibility, and the city administration has not confirmed whether the gas was the direct cause.
An aerial photo of the accident site shows emergency crews working around the open manhole.
Investigations are ongoing to understand the exact cause of the tragedy and whether safety protocols were followed properly during the inspection.