Those responsible for polluting the River Trent in Staffordshire with the chemical cyanide will be prosecuted, the Environment Agency has said.
Thousands of fish have died and people were warned to stay away from the river while the pollution, between Stoke-on-Trent and Yoxall, is controlled.
The agency said it aimed to prosecute under the Water Resources Act.
A spokeswoman would not give more details of where the cyanide came from for legal reasons.
Environment officers, who began pumping oxygen into water on Tuesday night, have managed to greatly reduce levels of pollution on the 30-mile (48 km) stretch, the agency said.
The public, farmers, anglers, boaters and animals have been warned to stay away from the water as a precaution.
Sewage in water
Severn Trent Water said it did not get its water from the Trent and the water supply had not been affected "at all".
The firm said the cyanide had been released into the sewers upstream from the company's Strongford sewage treatment works in Stoke.
That had caused problems at the works, which meant partially-treated sewage was also released into the river.
Simon Cocks, waste water services director, said Severn Trent was not linked in any way to the chemical but said it was working with the Environment Agency to find out what had happened.
He said: "We are deeply concerned about the impact this chemical pollution has had on our sewage treatment system and the community in which we operate."
Alan Walters, of the agency, said the incident had had "a huge impact".
However, the agency said it had not received any reports of humans or pets being affected by the contaminated water.
Environment officials were tracking the pollution as it moved downstream and would keep taking water samples to monitor the situation.
Meanwhile, Michelle Bloor, a toxicology and aquatic pollution expert from Portsmouth University, warned the whole local ecosystem could be affected.
"Cyanide is a highly toxic substance. It has obviously come into the water from somewhere and it can leach through soils," she said.
"Any contaminants entering a river is a cause for concern because of the impact on wildlife. It will change the environment of the river."
The Water Resources Act makes it an offence to allow poisonous or polluting matter to enter rivers. There is no limit on the fines which can be imposed if the case leads to a conviction at crown court.