A fish kill in a County Londonderry river was caused by 30,000 gallons of slurry, the agriculture minister has said.
It happened in the Grange River, a tributary of the Moyola, on Thursday afternoon.
Members of the local angling club said a significant stretch of the river near Desertmartin had been affected.
NI's Environment Agency was alerted of slurry and dead fish in the water.
Inspectors who went to the scene confirmed the fish kill and traced the source.
"An NIEA Inspector is currently working with the site owner to try to prevent further run off from the premises," a spokesperson said.
"NIEA have collected statutory samples and the joint investigation with DAERA Inland Fisheries is ongoing."
On Friday, Agriculture Minister Edwin Poots told a Stormont committee that the kill was caused when an over-ground slurry tank collapsed and 30,000 gallons of slurry ended up in the river.
But the NIEA later suggested a collapsed over-ground tank was not the cause of the spill.
"Investigations are ongoing but at this stage it is not believed that the incident was the result of a structural failure of the tank," said a spokesman.
The Moyola is a 27km-long river that cuts through the heart of mid-Ulster, rising in the Sperrins and flowing into Lough Neagh.
It is considered one of Northern Ireland's best salmon rivers and is also home to dollaghan and brown trout.
An attempt will be made later to count the dead fish and assess the environmental impact of the incident.