"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 on Thursday.
Battalion Fire Chief Richard Rugg said rescuers recovered the bodies Thursday night. Two of the victims are believed to be employees of the construction company and the other two were contractors working with them, authorities said.
Federal inspectors were being sent to the scene Friday, authorities said.
Joseph Kimmes II, the founder of J. Kimmes Construction, which owns the Lakehead Blacktop and Materials landfill, said one of his sons was among the dead. He would not say who the other victims were. Four of his sons took over the company in 2005.
The accident happened in a large collection tank that held drainage and leeched water from a landfill used for demolition projects, he said. The water collected in the tank eventually gets pumped into the city sewer system, he said.
Fire Chief Tad Matheson said it contained water and fumes of hydrogen sulfide, which commonly is released with sewer products and is highly dangerous. The victims were believed to have died before rescuers arrived, Matheson said
Firefighters with breathing equipment led the recovery effort, and workers pumped water out to help.
The head of the Russian Finance Ministry, Anton Siluanov, said that the Americans would suffer additional losses if they impose sanctions on Russia's public debt