Detectives believe that at least six men, including some who were dressed in women's clothes and Swat team uniforms, were involved in a deadly shooting at a boxing weigh-in.
The Gardai said there were three gunmen, armed with automatic weapons and wearing Swat team uniforms, who burst into the Regency Hotel in Dublin on Friday.
Bullets were fired as up to 300 sports fans including children gathered ahead of a world title fight between Jamie Kavanagh and Antonio Jao Bento leaving one man dead and two more injured.
Another two gang members, who were armed with hand guns, included a man who was disguised as a woman in an auburn wig and another who was stocky and wearing a beige cap.
A sixth man remained in the driver's seat of the silver Ford transit getaway van, detectives said.
Bystanders ran for their lives as the gang opened fire in the lobby.
The victims, aged in their 20s and 30s were taken to hospital where a man in his 30s, who was "riddled" with bullets, later died.
Gardai Chief Superintendent Barry O'Brien said it was a "particularly nasty incident" that had involved "severe weapons".
He said: "Two individuals were wearing what was described as police-type uniforms. In that regard it was described as a Swat uniform with metal helmets, similar to what you see on crime dramas."
Bystanders in the lobby of the Regency Hotel on the Swords Road, around two miles north of Dublin city centre, described cowering for their lives as the attackers fired multiple rounds at the men.
Kevin McAnena, a sports reporter who was covering the event, said one of two gunmen pointed a "big, massive" gun at him while he sought cover behind the reception desk.
Mr McAnena said: "I was shouting even before he came over, 'don't shoot, don't shoot' ... I've never felt terror like it."
Mr McAnena said he saw a gunman firing into a victim's leg.
"The man who is now dead - he was no more than six foot from me.
"The gunshots I heard after I jumped over the receptionist's desk were probably the gunshots that finished him off.
"That same gunman looked over at me before pointing the gun at me, telling me, I think he said 'get up' or 'go away', or something like that."
Mr McAnena heard two shots outside the hotel and at least three more inside.
"They were big, massive guns – gardai have told me since they were AK-47 rifles," he said.
"It was just so surreal, it happened so quickly."
In dramatic amateur video of the attack, rapid gunfire can be heard erupting in the lobby just seconds after the men left the stage in a function room at around 2.30pm.
One child is heard crying: "Daddy, help me. Daddy, what was that?"
As chairs are tossed aside and people dive for cover, one petrified woman is heard saying "What the f*** is going on?"
Some spectators fled from the room while heavy rock music continued to play through speakers.
Witness Mel Christle, president of the Boxing Union of Ireland, was standing on a podium where the last of the boxers weighing in had just stood down when the gunfire erupted outside the room.
He said: "There was an incredible noise out there (in the lobby) and that is where I saw a body, a corpse, lying literally at the edge of the reception desk."
It is believed a number of gangland figures were expected to be attending the weight-in.
Speculation has centred on a feud between Dublin gangsters and an Irish criminal outfit based in the south of Spain.
The boxing chief was adamant the attackers were not targeting the boxers gathered for the weigh-in, but other individuals who were hit in the hotel reception.
The gunmen fled the attack in a van, which was later found set alight nearby.
Saturday's World Boxing Organisation (WBO) lightweight title fight - billed as Clash Of The Clans - was cancelled.
Kavanagh later tweeted: "Anyone asking I'm OK! Thanks you for asking. I was lucky today is all I can say....."
Assistant Commissioner Jack Nolan said: "We know that there were a considerable number of people present in the Regency Hotel and we are appealing to them to provide any information they may have in relation to the shooting."
Police also appealed for potential witnesses - including anyone who was in the area of the Regency Hotel between 2pm to 2.45 pm or who was in the Charlemont Estate at that time - to come forward.