Four workers have died in Japan after falling into a manhole near Tokyo as they inspected sewage pipes, authorities said.

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

The
workers – all men in their 50s – were checking pipes in the city of Gyoda north of Tokyo on Saturday (local time) 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.