(BSS/AFP) - Four workers have died in Japan after falling into a manhole near Tokyo as they inspected sewage pipes, authorities said Sunday.

The incident comes after a huge sinkhole swallowed a truck driver near the capital in January after a road collapsed because of corroded sewage pipes, sparking a nationwide inspection.

The workers -- all men in their fifties -- were checking pipes in the city of Gyoda north of Tokyo on Saturday when one fell down the manhole, followed by three more who tried to save him, the local fire department told AFP.

The department said rescuers detected hydrogen sulfide -- a gas toxic in high concentrations -- coming out of the manhole.

But city officials refused to be drawn on the cause of the initial fall.

"Detailed circumstances leading up to the accident are still unknown, so it`s too early for us to say anything about our responsibility," a Gyoda city official said on condition of anonymity.

The four workers were retrieved and taken to the hospital where they were pronounced dead, according to local media reports.

Around 10 workers were at the scene of the inspection, ordered to clean the pipes of wastewater and sludge if necessary.

In May Japanese rescuers recovered the body of the dead 74-year-old truck driver months after he was swallowed by the road collapse in the city of Yashio.