Gardaí are tracking down members and associates of a gang suspected of the murder of Declan O’Reilly in Dublin on Monday night.
But Garda bosses said that while this was one of their "main" lines of inquiry, it was not the only one.
Meanwhile, detectives in Laois are trying to gather evidence in connection with the murder of Gerard Eglington at his home in Portarlington on Monday morning.
Eglington, 27, was a senior member of a gang in the Crumlin-Drimnagh feud in south Dublin with a rival outfit, which has claimed 15 lives to date.
Garda bosses said there was no connection between the two murders.
Mr O’Reilly, 32, was walking home with his 12-year-old son along South Circular Rd at 8pm when he was shot dead. Gardaí said a man got out of a silver BMW, ran over, and fired four or five shots into him.
The killer, aided by a driver, then escaped. The car was found burned out in the nearby Tenters area. Mr O’Reilly, from Parnell Rd, Crumlin, had been walking to his partner’s home, also in nearby Crumlin.
Gardaí are investigating whether Mr O’Reilly was targeted in retaliation for the violent death of a prisoner in Mountjoy in Jun 2007.
Mr O’Reilly was acquitted in Feb 2011 of the murder of Derek Glennon, a violent criminal and drug dealer. Three days after he was acquitted, his sister’s house in Crumlin was targeted in a grenade attack.
In Sep 2011, Mr O’Reilly survived an attempted murder in Harold’s Cross, where he was hit three times with a low-calibre gun.
Associates of Glennon are known drug dealers in the Dolphin’s Barn, Bluebell, and Drimnagh areas of the city. Gardaí are now tracking members of that gang and their associates.
Supt Thady Muldoon, who is leading the investigation, yesterday said it was "certainly" a line they were examining.
"Given the history of Mr O’Reilly, that would be one of the main lines of inquiry, but not the sole avenue."
He said Mr O’Reilly had been "fully aware" there was a threat to his life and had been given security advice by gardaí.
Supt Muldoon appealed to anyone who saw the BMW 3 series, registration 04 D 48695, to contact Kevin St Garda Station on 01 6669400. He said it was stolen last week and may have been parked up since. A firearm was found in the car but was badly burned.
Assistant Commissioner Nacie Rice said: "It is quite obvious there is a very, very small number of people who are so ruthless that they have no concerns for the rule of law, less concern for children or anyone else."
In the Dáil, Taoiseach Enda Kenny said human life had become "very cheap indeed".
Responding to Fianna Fáil leader Micheál Martin, who questioned whether the gardaí were sufficiently resourced to deal with serious criminals, Mr Kenny said the Garda Commissioner would be given "whatever resources are necessary" to bring those guilty to justice.
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