A large cat-like animal has been spotted roaming wild close to where sheep have been mauled.
Gardai are awaiting the results of laboratory tests on droppings found close to the dead livestock to determine the identity of the mystery killer.
Yesterday, they released a photograph of the creature, which was taken by a woman in the Ballylawn area of Manorcunningham, Co Donegal.
"It is taken from a distance, so it is impossible to be certain, but the woman who took the picture saw this animal up close in her garden and she knows it wasn't a dog," said Supt Vincent O'Brien.
He confirmed that gardai had met with the local ISPCA inspector and the county's wildlife officer as concerns began to grow about the animal.
"We have got dropping samples and they've been sent away for analysis, so we're not exactly sure what kind of animal we're dealing with. However, the sightings report that it was a large, black, cat-type animal such as a cougar, that moved extremely fast across the fields, and was noticed crawling on its belly," he said.
He added that the latest mystery animal did not fit the description of another sighting in the same area a year ago.
"The one last year was brown and white. It was sighted a number of times and then it fizzled out.
"This one is described as black. We are asking people to be vigilant and anyone who sees it to report it to their local garda station ."
He confirmed the matter was now being treated "extremely seriously" by gardai and a full-scale search was under way to find the animal.
"We're not making any further comment at this stage and we don't want to alarm people in any way and create a panic," he said.
"We'd particularly appeal to any owner of exotic animals that may have escaped recently to contact us immediately as part of our inquiries."
The cougar or wild cat may have been dumped in Donegal by a circus crew or escaped from a zoo.
Last year, a major hunt got under way in the North for a black panther along the north Antrim coastline. But it was called off when it was decided the animal had disappeared into the wild.