About 10,000 fish have been killed by an agricultural pollutant, the Environment Agency have said.
An investigation revealed roach and trout died in the River Mole, South Molton.
The Environment Agency said the source of the chemical-based pollutant which impacted 5km (3.1 mile) of a river had been identified and it would now be working out how it got into the river.
The results of the investigation should be released later this week.
There is no longer a danger of the pollution killing any more wildlife in the river because the officers on site got as much pollution out of the river as possible, the Environment Agency said.
It is continuing to restore oxygen levels to the river to protect wildlife.
Thousands of fish have also died in the River Sheppey in Somerset after a pollution spill.