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 Northern Ireland'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'."