Fish stocks on a stretch of the River Trent in Staffordshire that was polluted with cyanide appear to have been wiped out, experts have said.
Investigations are continuing into the source of last week's chemical leak near Stoke-on-Trent.
The Environment Agency said a survey along a 700m section near Stone on Wednesday had found almost no evidence of living fish.
Stocks several miles further downstream appear relatively unaffected, it added.
The agency said further tests were being carried out on river life and later this week sediment samples would be taken for more detailed analysis.
The pollution, which affected the river between Stoke-on-Trent and Yoxall, was discovered on 6 October.
Environment officers were able to greatly reduce pollution levels on the 30-mile (48 km) stretch by pumping oxygen into the water.
The public, farmers, anglers and boaters were warned to stay away from the water as a precaution and keep animals away.
Severn Trent Water, which was not linked in any way to the chemical, said the cyanide had been released into the sewers upstream from the company's Strongford sewage treatment works in Stoke.
That caused problems at the works and resulted in partially-treated sewage also being released into the river.
The Environment Agency said it would prosecute those responsible under the Water Resources Act which 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.