Manhole workers likely died from hydrogen sulfide poisoning, asphyxiation. The male workers, all in their 50s, were inspecting a sewage pipe in Gyoda City on Saturday. Police say they believe one of them fell in first, and the rest followed while trying to rescue him. All of them were later confirmed dead.
Inspections on the bodies suggest the first man and one other worker died after inhaling hydrogen sulfide. They indicate the other two also suffered poisoning from the toxic gas and died from asphyxiation.
The company conducting the inspection work explained that no alarm sounded from the device used to detect hydrogen sulfide levels when one of the workers first entered the manhole.
But it said a concentration exceeding 150 ppm was detected shortly after the four men fell in.
The figure is more than 15 times the rule permitted limit.
The company also said none of the workers were wearing fall protection gear or protective masks at the time of the accident.