A single gunman murdered a 36-year-old Dublin man in north Belfast on Saturday, police have said.
Robbie Lawlor was shot multiple times outside a house on Etna Drive in Ardoyne at about 11:50 BST.
Mr Lawlor was a suspect in the murder of a 17-year-old in County Louth in January. It is understood he was a senior figure in a feuding criminal gang.
Three men and a teenager have been arrested over the killing.
The men are aged 27, 30 and 33, while the teenager is 17 years old.
Three were detained on Saturday - the fourth was arrested on Sunday.
The police have described the killing as "brutal".
On Sunday evening, PSNI Det Supt Jason Murphy said the murder weapon had not yet been recovered, although two properties, one in west Belfast and one in Crumlin, were searched.
Mr Murphy said he did not believe Mr Lawlor was in Ardoyne on Saturday "by accident".
"I believe he had some reason to be there and key lines of enquiry for me, at this stage, are to establish why he was there and what his connection to the address is," he said.
Mr Lawlor was well known to gardaí (Irish police) for his involvement in serious and organised crime, according to RTÉ (the Irish broadcaster).
RTÉ said he had more than 100 previous convictions and had been warned by gardaí that his life was in danger before he travelled to Belfast.
He was also a suspect for at least three other gangland murders.
Det Supt Murphy said he was keen to hear from anyone who was in Etna Drive between 11:00 and 12.30 BST on Saturday, and who may have witnessed the shooting or the gunman escaping.
He also appealed for help in tracing the movements of a VW Sirocco car, registration YLZ 7052.
"I believe it was in Estoril Park on Friday and was burnt out in Kingston Court immediately after the murder yesterday," he said.
Neighbours had previously described how they tried to save Mr Lawlor.
One man said he "heard four big bangs and ran outside to see what was happening".
He said he saw him "stumbling out of the garden" and that he ran over to help him after he collapsed.
"I tried doing CPR but I couldn't save him," he said.
Another witness said she ran to help but that Mr Lawlor was already dead so she put a sheet over him to "give him some dignity".