Police and military chiefs have been ordered to disclose intelligence files in a major legal action over alleged collusion with loyalist terrorists behind the Miami Showband massacre.
A lawyer representing survivors and relatives of murdered group members confirmed the High Court has directed that more than 80 categories of documents are to be made available.
They are understood to include material held on Robin 'The Jackal' Jackson, a notorious UVF commander and suspected RUC Special Branch agent linked to scores of murders.
Following the development, solicitor Michael Flanigan said: "This is a case in which collusion is self-evident.
"The documents which the court has ordered disclosure of will go some way to explaining how that collusion came into effect, resulting in the loss of the lives of these innocent, talented young men."
Victims of the atrocity are suing the Ministry of Defence and the PSNI over alleged collaboration between serving soldiers and the paramilitary killers.
Three members of the popular band were taken from their tour bus and shot dead on a country road after a gig in Banbridge, Co Down in July 1975.
They were travelling home to Dublin when a fake army patrol made up of UDR and UVF members stopped them at a bogus checkpoint outside Newry.
Band members were made to line up at the side of the road while attempts were made to hide a bomb on the bus.
The device exploded prematurely, killing some of the would-be bombers.
The other gunmen then opened fire on the band, murdering lead singer Fran O'Toole, guitarist Tony Geraghty and trumpeter Brian McCoy.
Two other band members, Des McAlea and Stephen Travers, were also injured but survived the atrocity.
In 2011 a report by the Historical Enquiries Team raised collusion concerns around the involvement of an RUC Special Branch agent.
It found that UVF boss Jackson, a one-time UDR member who died in 1998, had been linked to one of the murder weapons by fingerprints.
Jackson claimed in police interviews he had been tipped off by a senior RUC officer to lie low after the killings.
He went on trial charged with possession of a silencer attached to a pistol used in the murders but was subsequently acquitted.
Two serving members of the UDR were, however, eventually convicted for their part in the attack.
Based on documents uncovered by campaign groups, writs have been issued against both the MoD and Chief Constable.
Damages are being sought for assault, trespass, conspiracy to injure, negligence and misfeasance in public office.
Military chiefs allegedly knew about but failed to stop loyalists infiltrating the UDR's ranks, according to the victims' case.
They also claim police are liable for vetting carried out on applications to join the army regiment and the use of agents such as Jackson.
Despite securing the order for discovery, the defendants could still try to retain some files.
Any application for Public Interest Immunity would be based around the potential damaging impact of full disclosure.