The search for the source of pollution which killed thousands of fish on Teesside is being narrowed down.
A stretch of the River Tame and its tributary, Main Stell, near Stokesley, south of Middlesbrough, was affected.
The Environment Agency said hundreds of trout, lamprey and eels had been affected and thousands of stone loach and minnows.
The pollutant is odourless and fairly colourless and the agency is warning people to stay away from the river.
An Environment Agency spokesman said eels were usually not affected by pollution, and the fact they had been indicated it had been a fairly high concentrate of pollutant.
He said they did not think the Leven, which the Tame feeds in to, had been affected.