Two men in Ojiya, Niigata Prefecture, died Saturday as they were trying to remove mud that had filled a well as a result of last month's heavy rains, police said.
The two were identified as Takashi Sato, 39, and Kizo Kawai, 65.
Carbon monoxide was detected in Sato's blood, and a pump using a gasoline-powered engine was placed near the well. Police said they suspect carbon monoxide from the engine found its way into the roughly 6-meter-deep well.
The two men and Sato's father, Seikichi, 69, started pumping mud and debris out of the well, which was used for irrigation, at around 8 a.m. Sato first collapsed at around 10 a.m., and Kawai, who entered the well to rescue him, also lost consciousness, police said.
The bottom of the well was filled with mud and sand, over which there was about 50 cm of water, police said.
The well is in a rural area about 1.5 km from the center of the city.