A farm is suspected as being the main source of pollution, thought to be slurry, in a river where a number of fish died this summer, bosses at the Environment Agency (EA) say.
Environment minister Emma Hardy wrote to Crewe and Nantwich MP Connor Naismith following the incident on the River Weaver in the Nantwich area.
At about 13:30 BST on 21 August, a member of the public alerted the agency to fish in distress, she said in the letter.
The minister added the EA “is confident that it has identified the principal source of the pollution of the most recent fish death, and a full investigation is under way”.