Garda Commissioner Martin Callinan warned the murderers of Garda Adrian Donohoe, "Keep looking over your shoulder."
He also warned anyone who is withholding information on the killers that "now is the time to come forward to come forward, before we start knocking on doors".
At a press conference in Dundalk Garda station to mark the first anniversary of the shooting dead of the detective garda at Lordship Credit Union in Bellurgan, Co Louth, the commissioner described the investigation as "huge and very complex".
He revealed that in the past 12 months the investigation team has taken over 2,100 statements, gathered 400,000 hours of CCTV footage, collected over 1,200 exhibits and carried out more than 30 searches under warrant.
To date nobody has been charged in relation to the brutal murder of the 42-year old garda on the evening of January 24th, 2013, when he was gunned down in the car-park of the credit union while providing an armed escort for the staff who were carrying cash for lodgment.
Several of the main suspects in the five-man gang fled the country in the aftermath of the shocking murder, and the Garda Commissioner confirmed that officer have travelled to several countries to gather intelligence and evidence, and he thanked the FBI, the PSNI and the Dutch police for their help in the investigation.
"We believe we know the identities of the people who carried out this crime," he said, adding' "we are keeping a very close eye on the target".
Responding to a suggestion that some people may be too frightened to give information to the Gardai urged anyone with information on the killing to come forward, Mr Callinan had a message for them: "Come forward and help us, we will make sure no harm comes to you," he said, insisting that "We will leave no stone unturned in terms of protecting them".
The commissioner added that anyone who may be threatening people who wished to help with the inquiry would be dealt with "very very severely indeed".
Although he refused to give a date when arrests would be made, he said "we are following all lines of inquiry and we will continue to be as robust and committed as we were a year ago".
An anniversary mass will take place later this evening in the Redemptorist church in Dundalk where Garda Donohoe's state funeral was held last year. Large crowds are expected to attend the service, including his wife Caroline and children Amy and Niall. A candle-lit vigil in his memory will be held tomorrow evening at the spot in Bellurgan where he was murdered.
By Lise Hand