Garda Commissioner Martin Callinan has warned the killers of detective Adrian Donohoe - keep looking over your shoulder.
On the eve of the first anniversary of the Dundalk officer's death, the Garda chief said the force was keeping a close eye on the suspected murderers.
"I say to the people involved in this particular crime - keep looking over your shoulder," he said.
Father of two Garda Donohoe was gunned down during a botched credit union robbery near his home in Lordship, outside Dundalk, Co Louth, on January 25 last year.
The detective was escorting a cash delivery when he stepped out of his patrol car and was shot in the head by one of a four-man criminal gang, believed to based along the Irish border.
The killers fled the scene with just 4,000 euro and dumped their getaway car in Keady, Co, Armagh, across the border in Northern Ireland.
Mr Callinan, the Republic's most senior police officer, said the force is satisfied they know the identities of the murderers.
"Suffice to say we are keeping a very good idea on the target," he added.
The mammoth investigation has seen officers travel to Belfast, the Netherlands and the United States.
"We have sent officers to a number of these countries to try gather intelligence and indeed evidence," said Mr Callinan.
The Garda boss praised the "invaluable" help given to the murder investigation team by the PSNI, the FBI and the pan-European law enforcement agency Europol.
A year on since the killing, there have been 4,000 lines of inquiry, mostly completed at this stage, a painstaking trawl through 400,000 hours of CCTV footage and 2,100 statements taken.
There have also been 1,200 exhibits gathered and more than 30 searches carried out under warrant.
Mr Callinan said there were people on both sides of the Irish border who have intelligence and information about the killers that could speed up the conclusion to their inquiries.
"These are the people who I say are shielding the murderers," he said.
"These are the people I'm making a heartfelt plea to, to come forward and help us so we can progress the investigation even faster."
While there are a variety of reasons why some witnesses were reluctant to come forward, Mr Callinan vowed to make sure no harm would come to anyone who assisted the probe.
The Garda chief also insisted no stone would be left unturned in bring the killers to justice, with detectives being handed whatever they need to help the pursuit.
"There is no question of resources being wound down in terms of the work that is outstanding and the work that needs to be done," he added.
"Nothing will be left undone that needs to be done."
An anniversary Mass in memory of Garda Donohoe will be held this evening at St Joseph's Redemptorist Church in Dundalk.