It happened in the Muff River at Eglinton on Friday morning.
The NI Environment Agency (NIEA) and the Loughs Agency are carrying out a joint investigation into the pollution.
It described it as a “major fish kill” and said a “specific line of enquiry” had been established.
After the alert was raised, the NIEA deployed an inspector to assess the environmental impact of the pollution.
It said it could not provide any further details as it would risk “compromising any subsequent action”.Tags: