The pollution of a County Tyrone river by a slurry spill has been described as a "catastrophic event" by the Loughs Agency.
A concrete store ruptured, pouring more than 100,000 gallons of slurry into a waterway which leads to the Owenreagh River near Dromore.
The Loughs Agency estimates up to 15 miles of water has been affected.
The Drumragh River, which the Owenreagh runs into, is an important salmon and trout spawning ground.
Specialist equipment is being used to aerate the water in an attempt to mitigate the impact.
Samples have been taken with a view to prosecution. The Loughs Agency is investigating, supported by the Environment Agency.
The incident was reported to the agency on Wednesday and the source had been identified. Some dead fish have been found.