Environmental officials are trying to establish the source of a devastating pollution incident that killed more than 1,000 fish in a Northern Ireland river.
Several kilometres of a tributary of the River Faughan at Claudy, Co Londonderry has been contaminated with suspected silage effluent.
A range of fish species died in the polluted waters.
The Department of Agriculture, Environment and Rural Affairs (DAERA) said officials from the Loughs Agency and the Northern Ireland Environment Agency (NIEA) had been on site since the incident was first reported on Monday night trying to establish the origin of the pollution.
A DAERA spokesman said: "Staff from Loughs Agency, who are leading this investigation, are working closely with NIEA to quantify the number and species of fish affected, and the cause, which is suspected to be silage effluent.
"The latest available figures are that in excess of 1,000 fish of a range of species have been killed over several kilometres of river. Assessment of the fish kill is still ongoing."
East Londonderry MP Gregory Campbell called for an urgent investigation.
"This is a major pollution tragedy on the Faughan which is a well-known salmon and trout river in the North West for anglers," said the DUP representative.
"The wider implications on this major fish kill for future generations of fish stock cannot be underestimated and it is imperative that the source of the pollution is found as soon as possible to minimise further losses."
Sinn Fein Assembly member Caoimhe Archibald called for tough penalties for anyone found guilty of polluting a river.
"The penalties imposed for incidents such as this should be as severe as the law allows," she said.
"At times the fines are not a sufficient deterrent and do not match the damage being done to the environment."