At least 10 people died on Friday during sewage treatment works outside Russia's southern city of Taganrog when they were overcome by methane, officials said.
Local authorities said in a statement the accident took place at a pumping station in the village of Dmitriadovka outside Taganrog, with 10 workers dead and eight in hospital.
Russia's Investigative Committee, which probes major crimes, said the workers had been exposed to a noxious substance and a criminal investigation had been launched into potential safety violations.
Regional authorities said in a statement the workers had been exposed to methane, which among other gases is a by-product of human and organic waste.
A spokesperson for the regional branch of the Investigative Committee told AFP that bodies were still being extracted from the area and the final death toll was not yet confirmed.
Investigators and senior regional officials were dispatched to the scene.