Wisconsin ? Toxic fumes are believed to have killed four workers whose bodies were pulled from a 12-foot-deep sewer line near a construction company landfill, authorities said Friday.

“They were either down in the manhole trying to clear a blockage or they were going down to repair a pump in the bottom of the manhole,” said Douglas County Sheriff Thomas Dalbec, who said he believes they died one after another as each went down to help Thursday.

Firefighters who responded to the scene got a hydrogen sulfide reading of 200 parts per million, Fire Chief Tad Matheson said. A safe reading of the toxic gas, which commonly is released with sewer products, is less than one part per million.