The agency said it was “following a definite line of inquiry” to determine the cause of the kill, which may result in legal proceedings being initiated.
Noel Carr, secretary of the Federation of Irish Salmon and Sea Trout Anglers, told BBC News NI that the fish kill was “soul-destroying”.
“This has set the river back by about 10 years now,” Mr Carr said.
“You’re talking about two-life cycles of salmon lost just like that.”
Mr Carr said the river was under a conservation order and was currently in the process of recovering.
He described the sheer amount of fish killed as incredibly concerning and hoped the IFI investigation will discover the cause of the kill as soon as possible.
