Gardaí investigating the callous murder of Detective Garda Adrian Donohoe are liaising with authorities in the United States and the UK in order to catch his killers.
The 41-year-old father-of-two was shot dead in the line of duty during an armed robbery at the Lordship Credit Union in Bellurgan, near Dundalk, Co Louth, on the morning of January 25, 2013.
No one has ever been brought to justice in relation to the incident and the gang of five involved in the attack escaped the scene with just €4,000.
On the fourth anniversary of Det Gda Donohoe's murder, senior gardaí yesterday made a fresh appeal for anyone with information in relation to the murder to contact them.
"Over the last four years, investigating officers have carried over 5,600 individual enquiries, taken over 2,500 witness statements and examined in excess of 20,000 hours of CCTV footage," Assistant Garda Commissioner Michael Finn said.
Chief Supt Sean Ward, of Dundalk garda station, said that An Garda Síochána was working with police forces in the US and UK as part of the murder investigation.
"We are engaged and working closely with police forces in the United States, in Europe and with colleagues in the Police Service of Northern Ireland, and as I said at the outset, our objective is that no stone will be left unturned in terms of bringing those responsible to justice," he said.