The families of those killed have released conclusions from a review, after cold-case investigators said it was "deeply troubling" that British Army involvement in the attack could not be ruled out.
Three members of the Showband were killed on 31 July 1975, as they travelled back to Dublin after playing a gig in Banbridge, Co Down.
They were flagged down at a bogus army checkpoint at Buskhill, near Newry, in the early hours.
To the objective, impartial observer, disturbing questions about collusive and corrupt behaviour are raised.
HET report
"The HET review has found no means to assuage or rebut these concerns," the report added.
The gunmen, who were wearing UDR uniforms, instructed the band members to line up at a ditch and state their names and addresses.
Two of the attackers were killed when a bomb unexpectedly exploded as they placed it in the back of the band's van.
The remaining gunmen then opened fire on the Miami Showband - shooting Tony Geraghty eight times in the back, while Brian McCoy was shot nine times and Fran O'Toole was shot as he lay on the ground face up.
Other band members pretended to be dead in order to escape being murdered.
Hours after the shootings, the UVF released a statement which said the loyalist paramilitary organisation was "justified" in taking action and "the killing of the three Showband members should be regarded as justifiable homicide".
Jackson - also known as 'The Jackal' - was arrested and questioned about the massacre and he claims that, during a police interview, he was told to "clear as there was a wee job up the country I would be done for".
One of the survivors, bass player Stephen Travers, said: "The most alarming finding concerns the involvement of Robin Jackson, aka 'The Jackal' - a notorious UVF member.
"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 was arrested at an early stage in the inquiry, but was released without charge. He died in 1998, aged 50.
I think this is firm proof that he was an agent.
Paul O'Connor, from the Pat Finucane Centre
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.
Survivor Des McAlea said he was disappointed no one was ever charged with his attempted murder and he would pursue that with the Public Prosecution Service and the PSNI.
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 from England.
The families also said they want the issue of Jackson's involvement particularly to be pursued.
Sinn Féin MLA John O'Dowd said: "It was well known that Robin Jackson was an agent for the British state, that he was allowed to kill Catholics with impunity throughout Mid-Ulster and beyond and that some of these killings were actually facilitated by the forces of the state.
"This speaks volumes about the British state's involvement in the conflict and rather than claim, as they did, that they were impartial observers or some sort of peace keeper, they were in fact up to their necks in facilitating and possibly encouraging sectarian killings and much more.
The HET report has been described as "vindication" for the families of the Miami Showband members by SDLP MLA Dolores Kelly.
"It again confirms what many people suspected; that there were very serious failures in the RUC in terms of investigating serious crimes in the past.
"The enormous question about why Robin Jackson was allowed to carry out this terror and inflict so much pain on victims over such a long period of time must be answered by the State as we cannot help but think if he had been put behind bars some people's lives may have been spared," she added.
Alliance Justice spokesperson Stewart Dickson said he believes it serves as a "wake up call" for the Secretary of State to put in place talks on how to address the past.
"The findings of this report are extremely troubling, though we have to acknowledge the reforms to policing that have been made since that time.
"I also believe that it is absolutely crucial that we have a comprehensive process in place to address the legacy of the past. We need an overarching strategy to help meet the needs of victims and survivors and help build a shared future," he added.
The HET report has been handed to the Police Ombudsman.