Dissident republicans have claimed they carried out a shooting in Dublin on Friday in which a man was killed and two others wounded.
The Continuity IRA (CIRA) said it also plans to carry out further attacks "on drug dealers and criminals".
Irish police said they were "keeping an open mind in relation to the criminal groups involved".
Three of the gunmen wore police-style SWAT team uniforms and were armed with AK47 assault rifles.
The police also said at least six people were involved in the attack at a weigh-in for a boxing match.
It came ahead of a Clash of the Clans boxing bout between Dubliner Jamie Kavanagh and Portuguese boxer Antonio Joao Bento.
The World Boxing Organization (WBO) European lightweight title fight was due to take place on Saturday night.
The man who died was David Byrne, 33, from Raleigh Square in Crumlin in the south of the city.
In a statement to the BBC, a man claiming to speak on behalf of the leadership of the Continuity IRA said its members were responsible.
A hard-line republican group, CIRA is violently opposed to any deal not based on a united Ireland. "Continuity" refers to the group's belief that it is carrying on the original IRA mission to force Britain out of Ireland.
The CIRA spokesman claimed Mr Byrne was targeted because he had been involved in the killing of Alan Ryan in Dublin four years ago.
Ryan, 32, was a leading member of another dissident republican group, the Real IRA. He was shot dead in September 2012.
At the time it was reported that Ryan's faction within the Real IRA in Dublin had been engaged in a feud with major gangland criminals.
"Although not a member of our organisation, we are not going to stand back and allow drug dealers and criminals to target republicans," the CIRA spokesman said.
'Not an isolated incident'
He also warned that further attacks were planned.
"This will not be an isolated incident," he said.
"Continuity IRA units have been authorised to carry out further operations. More drug dealers and criminals will be targeted.
"The Continuity IRA will carry out further military operations."
The Irish police, An Garda Síochána, have set up armed checkpoints in Dublin because of fears of retaliation for the attack.
The CIRA spokesman who issued the statement to the BBC provided a code word, but there is no way to independently verify this claim.
The nature of the attack, and in particular the use of AK47 assault rifles, are reminiscent of attacks carried out by the Provisional IRA and dissident republican groups.
But until now, the Garda have said they believe shooting was the result of a feud between major organised crime gangs.
Speaking on Monday after the attack, Sinn Fein president Gerry Adams said: "It was a brazen attack in broad daylight by criminal thugs who believe they can operate with impunity and above the normal rule of law.
"They should be locked up, where they belong."
Mr Adams added: "I understand a group calling itself the Continuity IRA has claimed responsibility for the attack at the Regency Hotel.
"They are not the IRA. The IRA are gone and their weapons are gone," he said.