Over 1,000 fish have beenkilled in a major pollution spill in the River Faughan.
The Northern Ireland Environment Agency was alerted to the incident near Claudy on Monday.
The Department of Agriculture and the Environment say the fish kill stretches several kilometres of water in County Derry.
Several species are reported to have been completely wiped out, although the source of the pollution has yet to be located.
It's still not known if the damage has been caused by organic material or chemicals.
MLA Gerry Mullan has called for a full investigation.
He said, “The news that in excess of 1000 fish near Claudy have been killed is deeply disturbing, with eyewitness reports that this was like something out of a horror movie.
"It is certainly one of the biggest incidents of its kind this year.
“It is essential that the Northern Ireland Environment Agency and the Loughs Agency carries out an immediate investigation and determines the cause of this disaster.
"Causing water pollution is a criminal offence and whoever is behind this – whether through negligence or malice – must be brought before the courts.
“With no idea what is behind this at this stage, it is unclear how long it will take before the stretch of water recovers.
"I will be seeking a full report from the Environment Minister on this shocking situation.”
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