Published Tuesday, 22 April 2014
The Oona River was the scene of the fish kill. (© UTV)
The Northern Ireland Environment Agency (NIEA) has classified the water quality in the Oona River, which is west of Dungannon, as "high severity" following the incident.
It is thought the majority of fish killed in the incident were brown trout and investigators have identified a potential source of the pollution.
The Environment Minister, Mark H Durkan, said: "I am concerned about this and I am keeping a close eye on the situation along with NIEA officials."
An NIEA spokesman added: "The water quality impact of this pollution incident is being classified as high severity.
"Investigating officers have identified a potential source of the discharge and statutory samples have been taken by NIEA.
"Investigations are ongoing in conjunction with the Department of Culture Arts and Leisure's Inland Fisheries."
© UTV News