The incident occurred on Saturday in the city of Gyoda, in Saitama Prefecture north of the capital, as the four men—all in their 50s—were carrying out an inspection with other co-workers.

City officials said the inspection was part of an emergency operation ordered by the central government, following a massive road cave-in in January. NHK reported that one of the workers fell into the manhole during the inspection, and three of his colleagues fell in while trying to rescue him.

According to police, the manhole measured 60 cm (24 in) in diameter and was over 10 metres (33 ft) deep. The local fire department also confirmed the incident to the AFP news agency. Video footage broadcast by NHK showed multiple emergency and rescue personnel gathered near the site. The fire department reported detecting hydrogen sulfide—a highly toxic gas—emanating from the manhole.

City officials declined to comment 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 told AFP, speaking on condition of anonymity.

The four workers were retrieved from the manhole and transported to hospital, where they were pronounced dead, according to local media. Around ten workers had been present at the inspection site, which included instructions to clean wastewater and sludge from the pipes if necessary.

In a related incident in May, Japanese rescuers recovered the body of a 74-year-old truck driver months after he was swallowed by a road collapse in Saitama Prefecture. (Al Jazeera)