Dúchas regional manager Paddy O’Sullivan refused to confirm, or deny, they had received such a letter.
“All we’re prepared to say is that we’re happy with the progress of the investigation and we’re helping the gardaí in every way possible,” he said.
Gardaí said they had not received a letter and were continuing with the probe.
The carcasses of the dead seals were found a week ago scattered on the stony shore at Beginish Island, close to the Great Blasket and about three miles from Dunquin.
The killing of the seals was widely condemned.
The seals are protected under the 1997 Wildlife Act. Post-mortem examinations carried out on the carcasses of three seal pups are believed to show one died from gunshot wounds, another from being clubbed and a third following a blow from a sharp object.
Tourism Minister John O’Donoghue, in whose constituency the killings occurred, said: “The full force of the law should be brought to bear on the culprits.”