The Environment Agency has verified that a pollutant entered a river and killed more than 100 fish.
The cause of the pollution in the River Bollin, close to Macclesfield, Cheshire, is still being looked into after foam started to appear on the water's surface.
Although investigations were "ongoing to understand the source," the agency claimed the pollutant entered the water at an outfall pipe close to Sutton Close.
It was "not related," according to United Utilities, to its local operations.
"We are aware of the circumstances surrounding this incident and have determined where the pollutant entered the river, but investigations are ongoing to understand the source," an Environment Agency (EA) spokesperson said. ".
According to the EA, environmental officers, fisheries experts, and ecologists are still on the scene to gather more data and assess the effects on the river.
They further stated that they were eager to speak with anyone who could help them identify the problem's origin and urged people to "remain mindful about what goes down the drain and into the drainage system, especially detergents or substances that could cause a hazard."