A man shot dead in Belfast on Saturday was 35-year-old Robbie Lawlor.
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.
He was fatally shot outside a house on Etna Drive in Ardoyne on Saturday.
Three men and a teenage boy have been arrested in connection with the killing, which police described as "brutal".
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.
Mr Lawlor was well known to gardaí 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.
On Saturday, Det Supt Jason Murphy condemned the Belfast killing and said Mr Lawlor had been shot a number of times at close range.
"Those involved in this murder selfishly gave no thought to the impact on frontline emergency services who are already working hard to keep people safe with an already stretched resource," he added.
"Not only did they carry out this callous murder leaving a family experiencing their worst nightmare, but they did not care that children and other members of this north Belfast community were placed at risk."
Mr Murphy said that a light coloured car, registration YLZ 7052, was found burned out in nearby Kingston Court, and he was trying to establish if this is relevant to the murder.
Neighbours 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".