An investigation has been launched after about 500 fish died in the River Dodder in Dublin.
Inland Fisheries Ireland said it has taken samples from the scene, around Old Bawn bridge in Tallaght, and are confident they have established the source.
But officials would not comment on the cause of the incident, which is believed to have been from a discharge of industrial waste or dumping.
A spokeswoman confirmed about 500 various types of fresh water fish were killed over a 600 metre stretch of the river.
The incident was reported on Friday evening.
"This was a significant fish kill," she said. "We are confident we know the source but we are continuing our investigations.
"We won't be releasing information on the source until we are absolutely certain of the results or after a prosecution."