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.
Read more: http://news.bbc.co.uk/2/hi/uk_news/england/staffordshire/8295402.stm