Garda Commissioner Martin Callinan has said he is satisfied the investigation team has identified all the gang members involved in the murder of Detective Garda Adrian Donohoe.
The detective was shot dead outside the Lordship Credit Union in Bellurgan, Co Louth a year ago tomorrow.
An anniversary mass was held in Dundalk this evening, as hundreds of people joined Caroline Donohoe to remember her late husband.
The pitch where he trained and mentored underage GAA teams has been named in his honour.
Making a statement outside Dundalk Garda Station this afternoon, the commissioner also said that the suspects are being monitored.
He warned them to keep looking over their shoulders because the gardaí would be knocking on their doors.
Mr Callinan said that the force would offer protection to anyone who is prepared to come forward and give information about the murder.
He said he understood that some people might be afraid, but he said people have to decide what kind of society they want to live in.
Members of the gardaí have travelled to the Netherlands and the US as part of the investigation, he said, adding that assistance had been received from police forces there and from Europol and the PSNI.
Mr Callinan said the murder of Det Garda Donohoe "had a profound impact not only on his colleagues in Dundalk, but also on the wider An Garda Síochána family.
"It had an impact not only on the lives of those he positively affected in Co Louth through his work, or through his involvement with the GAA community, but also country wide.
"Most importantly, his senseless, cold-blooded murder led to the bereavement of Caroline and their two children."
Det Garda Donohoe and his colleague were in an unmarked car as they waited at the credit union to escort staff and takings to a bank in Dundalk.
A gang of armed men were waiting in a car park outside to rob the credit union and moved to hold up staff as they were leaving the building.
The two gardaí got out of their car and Det Garda Donohoe was shot in the head as he approached the gang.
One hour after the shooting, a car matching the description of the one used in the incident was found burnt-out in Newtownhamilton, Co Armagh.
Mr Callinan said that Det Garda Donohoe's colleagues were investigating the killing with "the same dedication and thoroughness that Adrian provided throughout his career in An Garda Síochána".
He said: "Tens of thousands of man hours have been devoted to the investigation to ensure the perpetrators of this senseless act are caught.
"Throughout the course of the investigation there has been: over 4,000 individual tasks undertaken, over 2,100 statements taken, approximately 400,000 hours of CCTV reviewed, over 1,200 exhibits gathered ... more than 30 searches undertaken under warrant."
Earlier today, Drogheda mayor and former Garda Richie Culhane appealed to those involved in the murder to come forward.
Speaking on RTÉ's Today with Sean O'Rourke, Mr Culhane also claimed that there are people living in south Armagh who are hiding and protecting the identity of the gang involved in the attack and he pleaded with them for information.
Mr Culhane said sadness has not left Det Garda Donohoe's family, colleagues or the community since the incident.
He said: "The net is closing on these five people. There's absolutely no doubt about that. And what I would be saying to them ... of the five people one person pulled that trigger. One person shot dead in cold blood Detective Donohoe.
"I would be saying to any of those people come forward, come forward now with the information".