Inland Fisheries Ireland (IFI) has said it is preparing a file with a view to bringing a prosecution in relation to what it has described as a "significant" fish kill in Co Cavan this summer.
More than 1,000 fish mortalities were recorded in the Ballinagh River in the middle of July.
After being alerted to a contamination impact in the water by a member of the public, IFI environmental and fisheries staff identified deceased brown trout, stickleback, minnow and roach along a one-kilometre stretch of the river.
IFI has now said a suspected source of pollution has been identified and it is following a definite line of inquiry with a view to bringing a prosecution.
Separately, between 800 and 900 fish died in the lower reaches of the Mountnugent River and Captain's Bay area of Lough Sheelin in Co Cavan.
This fish kill took place days later, however, the incident at Lough Sheelin and the fish kill in the Ballinagh River are not believed to be connected.
The IFI has said the Lough Sheelin fish kill was caused by high water temperatures in combination with low levels of dissolved oxygen in the water.
In a statement, a spokesperson for IFI has said it is "investigating a significant fish kill in Co Cavan, with more than 1,000 fish mortalities recorded at Ballinagh River".
"Inland Fisheries Ireland (IFI) believe that between 800 and 900 fish died in Lough Sheelin in July.
"This incident at Lough Sheelin and the fish kill on the Ballinagh River in the same month are not connected, according to investigations executed to date by IFI.
"IFI is preparing a file with a view to bringing a prosecution in the Ballinagh River case, as a suspected source of pollution has now been identified.
"Separately, IFI's investigations suggest the Lough Sheelin fish kill was caused by high water temperatures, in combination with low levels of dissolved oxygen in the waterbody.
"IFI investigations of the site and on-site measurements did not detect a direct pollution source in relation to this incident," the spokesperson added.
The largest single fish kill on record here occurred after an unknown environmental irritant entered the water.
However, the inter-agency group set up to investigate the incident, last week concluded that it is unable to identify the exact cause of the fish kill.
