Ireland's police chief is liaising with senior detectives after a suspected dissident republican murdered an officer and shot a woman before turning the gun on himself.
Garda Tony Golden, a married father of three, was gunned down as he responded to reports of a domestic disturbance in Omeath, Co Louth yesterday evening.
The killer, Adrian Crevan Mackin - from the town, had been on bail awaiting trial for IRA membership.
The woman he shot is believed to have been his partner and is in a serious condition in hospital.
Garda Golden was 36 and had two daughters and a son, all under the age of eight.
He was originally from Ballina, Co Mayo, and had been stationed in Cabinteely, south Dublin, before a transfer to Omeath - a rural area a few miles from the border with Northern Ireland.
Garda Commissioner Noirin O'Sullivan was in Dundalk as the lead investigators held meetings over the case and she is expected to visit the crime scene in the Mullach Alainn estate.
Forensic examinations took place at the terraced house where the front door had been damaged and partly covered with an evidence bag.
Garda Golden's murder occurred in the same district where the last garda was killed on duty.
Detective Adrian Donohoe was shot dead on January 25 2013 as armed robbers held up the credit union in the village of Lordship a few miles away from Omeath, on the Cooley peninsula.
Garda Golden lived in the Blackrock area of Dundalk with his wife Nicola and their three children.
Tributes were led by President Michael D Higgins, members of government and the commissioner - who was also meeting the officer's widow.
"This most serious and tragic incident has not only led to the death of a member of An Garda Siochana, while on duty, but tragically has also involved the death of one and the serious injury of another person," President Higgins said.
"I want to express my deepest sympathy to the family and friends of the garda who has so tragically lost his life while responding to this incident. My thoughts at this time are also with all others who have been affected by the events in Omeath."
The gunman had been on bail awaiting trial for IRA membership at the Special Criminal Court in Dublin after being arrested at the start of the year following a joint Garda-Police Service of Northern Ireland operation to target dissident republicans in the border area.
Joan Burton, Ireland's Tanaiste (deputy prime minister), also expressed her sympathies to Garda Golden's widow.
"While we await the circumstances to emerge, our thoughts have to be with the relatives of the victims," Ms Burton said.
"In particular, I want to express remorse for the death of Garda Anthony Golden while carrying out his work. This has been a tragic and dark weekend. When such events occur, they put much of our daily news and concerns in perspective."
The commissioner said: "This incident highlights the unique nature of the job carried out by the men and women of An Garda Siochana, and the dangers they face every day."
Justice Minister Frances Fitzgerald said she was appalled by the loss of life.
"The fact that a garda has laid down his life while protecting the community is a cause of great sadness. His death will be mourned by the entire nation, grateful for the service which he gave to it," she said.
Door-to-door inquiries were also conducted in the Mullach Alainn estate as gardai attempt to piece together the events leading up to the shootings.
Garda Golden is the 88th member of the force to be killed in the line of duty and will be entitled to a State funeral.
Dermot O'Brien, president of the Garda Representative Association, said: "This dreadful, dreadful news is the deepest fear of every police family. This puts everything we do into perspective."
The leader of Ireland's Catholics, Archbishop Eamon Martin, spoke by phone from Rome to local priests and asked for prayers.
"Like me, they are very upset by this tragic incident which has devastated families and rocked the peaceful parish community in beautiful Omeath," he said.
Niall Collins, justice spokesman for the main opposition party in Ireland, Fianna Fail, said he was shocked and offered condolences to family and colleagues and the commissioner.
"Obviously, details of this incident are still emerging, but I am extremely shocked to hear that a member of An Garda Siochana has been killed tonight," he said.
Sinn Fein leader and representative for Louth, Gerry Adams, said he was shocked.
"My thoughts and sympathies are with the families of those who have died and have been injured," he said.
The Police Service of Northern Ireland also said thoughts were with counterparts in the Irish Republic and Chief Constable George Hamilton said: "Thoughts are with our Garda colleagues. An officer paying the ultimate sacrifice in the line of duty."