A manhunt is under way for a six-man gang who carried out a gun attack in the Regency Hotel in Dublin.
Detectives say they now believe three men dressed in SWAT uniforms stormed the hotel spraying bullets at a boxing weigh-in.
Two others - one disguised as a woman - remained outside with handguns.
A sixth man is believed to have remained in the getaway van.
Gardai have released an image of the van and are appealing for anyone with information about its movements in the area to contact them.
The brazen gangland shooting occurred as about 100 people, including several members of the Christy Kinahan crime syndicate, attended a weigh-in for a boxing event.
Hotel manager John Glynn saw one man shot dead in the reception area of the hotel.
"They fired into the bar and could have hit anyone. They could have turned on anyone," he said. "They could have fired on me or any of the staff or guests."
The hit is believed to be linked to the murder of criminal Gary Hutch, a nephew of Gerry 'The Monk' Hutch.
David Byrne (34), from Raleigh Square in Crumlin - a key Dublin-based member of the Christy Kinahan cartel for years - died at the scene.
Sean McGovern (30), from Captain's Road in Crumlin, was said to be in a serious condition in Beaumont Hospital after being shot in the stomach, while Aaron Bolger (25), from Killinarden in Tallaght, was being treated after he was shot in the thigh.
Taoiseach Enda Kenny called the shooting "an extreme case", while party colleague Richard Bruton branded it "a new low".
"There is a full-scale murder hunt going on here," said Mr Kenny. "Clearly, the interpretation here is that this is the work of rival criminal gangs in the Dublin area."
We're responding to a serious incident in a hotel in Whitehall, crews from Finglas, P'boro + advanced paramedics pic.twitter.com/5LshDt6SZ9— Dublin Fire Brigade (@DubFireBrigade) February 5, 2016
The Regency Hotel is on the edge of Mr Bruton's constituency. "I think it's very disturbing. I mean the Regency Hotel is a family-run hotel as you know - it's a place that many, many people go for a quiet evening or to events," he said.
"It is really very disturbing to see that level of wanton violence, the disregard for other people who were there.
"It's a new low I think in that sort of criminal activity."
Garda statement regarding shooting at Regency Hotel, Dublin. pic.twitter.com/s6d13Blpil— Niall O'Connor (@NI_ALLO) February 5, 2016
About 20 guests and tourists were in the foyer and bar area of the hotel as one man jumped onto the reception desk and pointed his gun at a BBC sports journalist, who begged not to be shot as he crouched down.
At the same time there was a shoot-out in the function room where 100 people, including dozens of children, had been.
Mel Christle, president of the Boxing Union of Ireland, said the weigh-in had finished when the drama unfolded.
"Everyone including quite young children - aged as young as five years old - and their relatives were there," he said.
Mr Christle said he saw two people - one a man dressed as a woman - run from the scene.
He also saw "One person who had been shot in his leg at the very back of the hall."
"When I was leaving about 20 minutes later, I could see a corpse slumped against the reception desk. His face was completely distorted, is what I can say about it."
Another witness, who was in the function room, said "all hell broke loose" when a man came in and fired three shots in the air.
Thanks for the tweets and other messages. I'm ok but unfortunately not everyone was as lucky.— Kevin McAnena (@KevinMcAnena) February 5, 2016
"There [was] glass breaking, chairs toppling over. It was pure chaos as people tried to run," he said. There were other guys in there with guns. They weren't shooting sporadically - they were picking their targets.
"Other men were armed but we didn't know if [they] were with the men pretending to be gardaí or shooting at them."
Video footage of the shooting later showed dozens of people fleeing in fear for their lives. Children can be heard on the recording screaming: "Daddy, help me… was that," as they ran outside for safety.
Journalist Kevin McAnena, who was in the foyer of the hotel as the gunmen entered, described how a "run of the mill" boxing weigh-in ended up with him "staring down the barrel of gun".
"Most other people had run out and I almost kind of froze," the BBC Radio Foyle sports reporter told RTÉ. "One other guy ran across the lobby and one of the guys with the guns shot him in the lower leg and he went down. He was possibly six feet from me. That's the guy who has now died. At that point I jumped over the receptionist desk and got on the ground."
"I started shouting 'don't shoot, don't shoot' because I could hear more gunfire from the other side."
Gardai said a short time after the four-man gang fled the scene a van was found burnt out at the Charlemont Estate in Marino.
Chief Superintendent Barry O'Brien, from Ballymun, appealed for witnesses. He said they were looking for four men, including some who wore "SWAT-type police uniforms with metal helmets, the likes of which you would see in a crime drama".
BY KEN FOY
The 34-year-old gangster shot dead in Friday's bloodbath in the Regency Hotel was one of the capital's most notorious gangland criminals.
David Byrne - from Raleigh Square in Crumlin - had been a key Dublin-based member of the Christy Kinahan cartel for years. He had been a chief target of specialist garda units.
Byrne - a first cousin of 'Fat' Freddie Thompson - was suspected of involvement in the importation of large amounts of drugs.
He was also believed to have been involved in attempting to smuggle rocket launchers into the country as part of the bitter Crumlin/Drimnagh feud.
His life of crime came to a brutal end when he was shot dead yesterday.
However, two of his close pals had a lucky escape when they were targeted by the brutal four-man hit team.
Sean McGovern (30) - who is from Captain's Road in Crumlin - is understood to be in a serious condition in Beaumont Hospital after he was shot in the stomach.
Aaron Bolger (25) - from Killinarden in Tallaght - is being treated after he was shot in the thigh.
All three men were friends and were known to gardai. None of the trio are suspected of being involved in the murder of Gary Hutch, which is being looked at as the main motive for yesterday's chaotic attack.
Bolger is understood to have spent a lot of time in Spain in recent years where he is involved with the MGM gym in Marbella. Daniel Kinahan also works there with professional boxer Matthew Macklin.
While McGovern has few previous criminal convictions, he has been a close pal of murdered man Byrne and Byrne's thug of an older brother - Liam - since they were children, according to sources.
Sean McGovern - who is said to be clinging to life - had been in a car business with Liam Byrne (38) and was often spotted in his company.
In March 2012, McGovern and Liam Byrne and another cousin of 'Fat' Freddie Thompson - Liam Roe - were arrested in Liverpool.
They were questioned about making death threats, blackmail and possession of housebreaking implements. They were never charged in that case.
David Byrne had been arrested numerous times by gardai over the years.
He was suspected of being involved in the murder of Gary Bryan in Walkinstown in 2006.
He was cleared of firearms charges in relation to a shooting incident connected to the Crumlin/Drimnagh feud, which took place in August 2008.
Officers from the Organised Crime Unit arrested Byrne - who was with Eugene 'The Devil' Cullen, a now-deceased convicted murderer - after a 20-year-old man was shot in the lower torso in Ballyfermot.
Byrne was also called to give evidence in the murder trial of Brian Rattigan, his gangland rival, in February 2009.
He claimed that he could not remember anything unusual happening outside Abrakebabra on August 25, 2001, when Rattigan stabbed Declan Gavin to death.