The gun attack on July 31, 1975, occurred as a minibus carrying the band was stopped at a bogus checkpoint outside Newry, Co Armagh.
The group was returning from a gig in Banbridge, Co Down, to Dublin.
Three men were killed: Tony Geraghty, 24, from Crumlin, south Dublin; Brian McEvoy, 32, a married father of two originally from Tyrone but living in Raheny, north Dublin; and Fran O’Toole, 28, a married father of two from Bray, Co Wicklow.
Fellow band member Stephen Travers was seriously injured in the attack, while a fifth member, Des McAlea, was also injured.
Details of a report into the massacre, carried out by the PSNI’s Historical Enquiries Team, were released by the families of victims and survivors at a press conference in Dublin organised by Justice for the Forgotten and the Pat Finucane Centre.
The internal special investigative unit said the murder was a pre-planned attack carried out by the UVF loyalist terror gang, including members of the Ulster Defence Regiment, part of the British army.
The gang had intended to plant a bomb to detonate at a later stage, allowing some elements to claim the band was carrying bombs for the Provisional IRA. However, the bomb detonated prematurely killing two terrorists, Harris Boyle and Wesley Sommerville.
The band members had already been told to get out of the bus and, when the bomb exploded, six other gunmen opened fire.
Three men were later convicted of murder.
The team raised particular concern about the possible involvement of a notorious loyalist terrorist Robert Jackson, known as The Jackal, after a gun linked to the atrocity was found to have his fingerprints on it.
There was no evidence that this information was passed on to the Miami investigation team. Jackson also claimed he was tipped off about the fingerprint evidence by senior RUC officers prior to being arrested.
No evidence of an internal investigation in these allegations was found.
Travers, the band’s bass player, who survived by pretending to be dead, said the finding was alarming.
“We believe the only conclusion possible arising from the HET report is that one of the most prolific loyalist murderers of the conflict was an RUC Special Branch agent and was involved in the Miami Showband attack.”
The HET found Jackson’s “stark” claims that he was told to lie low were passed on to RUC headquarters and the force’s complaints and discipline department but there were no records of any further investigation.
“To the objective, impartial observer, disturbing questions about collusive and corrupt behaviour are raised,” it concluded.
“The HET review has found no means to assuage or rebut these concerns and this is a deeply troubling matter.”
The report has been sent to the Police Ombudsman of Northern Ireland.
Mr Travers, Mr McAlea and families of the deceased all said they were “reasonably happy” with the report, although Mr McAlea said he wanted the PSNI chief constable and the DPP to tell him why nobody was charged with his attempted murder.
THE Miami Showband were one of the Ireland’s most popular live bands throughout the 1960s and 1970s.
The group, originally known as Downbeats Quartet, were formed in 1962 under Tom Doherty.
With seven number one songs, including There’s Always Me and Simon Says, the band enjoyed a large following as they relentlessly toured Ireland.
On July 31, 1975, the showband was returning from performing at a dance in Banbridge, Co Down, when their minibus was flagged down by men dressed in British army uniforms on the road to the border town of Newry.
Band members were told to line up in a ditch while UVF members posing as Ulster Defence Regiment members tried to plant a bomb on the minibus, which they hoped would explode later on as the musicians headed home to Dublin.
As the gang loaded the bomb, the musicians were asked for their names and addresses, but it exploded prematurely, killing UVF members Harris Boyle and Wesley Sommerville.
After the explosion, the UVF gang was ordered to open fire on the band, killing Fran O’Toole, Tony Geraghty and Brian McCoy. Stephen Travers was seriously injured yet survived by pretending he was dead, while the explosion blew Des McAlea clear of the immediate danger, and he escaped with scratches and severe shock.
Later that year Mr McAlea and Mr Travers reformed the band, but both men soon left, leaving the New Miami to tour, eventually disbanding in 1986.
The Miami name returned once again in 1996, this time fronted by Gerry Brown, brother of singer Dana.
The band performed on the 30th anniversary of the atrocity, with Mr McAlea and Mr Travers reuniting on stage at a Miami Showband Memorial Concert at Vicar Street in 2005.