Russian officials say 10 people have died of suspected poisoning during sewage treatment works in the southwestern region of Rostov.
Local authorities said in a statement that the accident took place on May 21 at a pumping station in the village of Dmitriadovka outside Taganrog.
Ten people died and eight others were hospitalized, they said.
The Investigative Committee said they were exposed to a noxious substance and a criminal investigation was launched into potential violations of safety rules.
According to preliminary information, the workers were exposed to methane, which among other gases is a byproduct of human and organic waste.