Police and military authorities have been ordered to release files relating to the Miami Showband murders.
Three members of the band were shot dead after a concert in Banbridge, County Down, in July 1975.
Belfast High Court ordered the release of more than 80 types of document on Wednesday.
They are understood to include material on Robin 'The Jackal' Jackson, UVF commander and suspected Special Branch agent.
A lawyer representing survivors and relatives of murdered group members said it was "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," he added.
Victims of the attack are suing the Ministry of Defence and PSNI (Police Service of Northern Ireland) over alleged collaboration between serving soldiers and the killers.
The band's lead singer Fran O'Toole, guitarist Tony Geraghty and trumpeter Brian McCoy were killed after their minibus was flagged down by loyalist paramilitaries, who were also members of the Ulster Defence Regiment (UDR), at Buskhill, north of Newry, in the early hours of the morning.
Two other band members, Des McAlea and Stephen Travers, were also injured but survived the attack.
In 2011, a report by the Historical Enquiries Team raised collusion concerns around the involvement of an RUC (Royal Ulster Constabulary) 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 eventually convicted for their part in the attack.
Based on documents uncovered by campaign groups, writs have been issued against both the Ministry of Defence (MoD) and the PSNI 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 regiment and the use of agents such as Jackson.
The MoD and chief constable could still try to retain some files on the basis of an application for public interest immunity.