Four men who died after falling into a manhole while checking a sewer pipe in Saitama Prefecture, north of Tokyo, likely suffered from hydrogen sulfide poisoning.

On August 2, one of the four workers fell into the manhole and the other three tried to help him but also fell.

Officials said autopsies showed that two men likely died of hydrogen sulfide poisoning and the other two may have suffocated to death due to effects of poisoning by the toxic gas.

The company the victims work for said the four were not wearing safety equipment to prevent falls.

It also said they were not equipped with gas masks even though the concentration of hydrogen sulfide in the manhole was more than 15 times the government standard.

According to the fire department, the hydrogen sulfide concentration detected in the manhole when firefighters entered it to rescue the men was eight times the standard level for conducting work.

The police are checking to see if proper safety management measures were in place when the men were working at the site.