Cold case detectives investigating the Miami Showband massacre could not rule out state collusion in the murders, families of the victims have said.
The Historical Enquiries Team (HET) also found it was “deeply troubling” that it could not rebut claims of British Army involvement.
Three members of the hugely popular band were killed in July 1975 at a bogus checkpoint set up on the main Belfast to Dublin road, near the junction with Bushkill Road in Co Down.
A UVF gang, including a number of serving Ulster Defence Regiment soldiers wearing Army uniforms, gunned down the musicians after a bomb they tried to attach to their minibus exploded prematurely.
Survivors and families of the victims have released conclusions from a review by the HET – a cold case unit investigating Troubles atrocities – which reports to the North’s chief constable Matt Baggott.
The review found: “To the objective, impartial observer, disturbing questions about collusive and corrupt behaviour are raised.
“The HET review has found no means to assuage or rebut these concerns and this is a deeply troubling matter.”
The families, who have not released the full report handed to them recently, also said the HET found an infamous loyalist leader was tipped off by police to lie low after his fingerprints were found on a weapon.
“The most alarming finding concerns the involvement of Robin Jackson, aka ’The Jackal’ – a notorious UVF member,” said Stephen Travers, one of the survivors.
“Jackson was arrested at an early stage in the inquiry, but was released without charge.
“The HET found disturbing evidence that Jackson was tipped off in May 1976 that his fingerprints had been found on a silencer attached to the Luger pistol used in the Miami murders.
“Jackson claimed that two RUC officers, one a detective superintendent, had advised him, in Jackson’s words, ’to clear as there was a wee job up the country that he would be done for’.”
It is believed the UVF's intention was to hide a bomb under the band's minibus which would explode on their journey back to Dublin, killing them all, and portraying them as transporting explosives for the IRA.
But the bomb detonated prematurely while being placed underneath the vehicle at the bogus checkpoint by Wesley Sommerville and Harris Boyle, both members of the UDR as well as the UVF.
All the band survived and the loyalist gang opened fire with automatic weapons at close range in an attempt to wipe out any witnesses.
Lead singer Fran O’Toole, guitarist Tony Geraghty, and trumpeter Brian McCoy all died.
Mr Travers, the bass player, was seriously injured but survived, by pretending to be dead, along with fellow band member Des McAlea, who was blown clear from immediate danger.
Responding to the findings, a clearly emotional Mr McAlea said it had been a long and winding road over the past 36 years for all the families of the victims as well as the survivors.
“Justice at last, hallelujah,” he said.
“It’s great that this day has come for all of us, it’s just so sad that it’s taken 36 years to get to where we are today.”
Three members of the UDR were convicted for the massacre.
Thomas Crozier, James McDowell and James Somerville all received life sentences, but were later released under the terms of the Good Friday Agreement.
Mr McAlea said he was disappointed no one was ever charged with his attempted murder and he would pursue this with the public prosecution service and the Police Service of Northern Ireland.
Both Mr Travers and Mr McAlea said one of the gang on the night was more authoritative than the others and spoke with a “posh English accent”.
Although the HET believes that man was McDowell, the survivors remain adamant the man was English.
The HET report has been handed to the Police Ombudsman in the North.
The families said they want the issue of Robert Jackson’s involvement particularly to be pursued.