IT is never too late for people to come forward with information on the murder of Detective Adrian Donohoe, the Garda Commissioner has said .
On the fourth anniversary of his killing during a credit union robbery, Noirin O'Sullivan said extensive inquiries continue to be carried out at home and abroad, including with the Police Service of Northern Ireland.
The Commissioner said the force is as determined as ever to bring those behind his murder to justice.
"This is a highly complex investigation and it is important that as with all such investigations that we make sure we have every single detail right," she said.
Mr Donohoe died after being shot as he confronted a gang holding up a small credit union near his home in Lordship, on the Cooley Peninsula in Co Louth, on January 25 2013.
The Irish League of Credit Unions has offered a 50,000 euro reward for information that leads to any arrest and prosecution.
The Commissioner praised the work of the murder inquiry and said it will not stop until justice is done.
"We cannot do this alone, we still need the public's help. Even the smallest piece of information could be vital," the police chief said.
"At this time, I would appeal to anyone with information on Adrian's murder to come forward and help us with our inquiries.
"There are still people out there who know who the killers are. It is never too late to do the right thing. Any information provided will be treated sensitively."
The Commissioner said Mr Donohoe was the epitome of all that is good about the force, dedicated to the community he served both professionally and personally.
Anyone with information on the murder should call the incident room in Dundalk Garda station on 042 9388470 or use the Garda Confidential Number on 1800 666 111.
Mr Donohoe was posthumously awarded the country's highest policing honour, a gold Scott Medal.
