Victims of the Miami Showband massacre are suing the Ministry of Defence and PSNI over alleged collusion with the loyalist killers, it was revealed today.
Writs issued by survivors and relatives of two murdered group members include claims for assault, trespass, conspiracy to injure, negligence and misfeasance in public office.
Both aggravated and the more punitive form of exemplary damages are being sought.
The High Court proceedings centre on the collaboration between terrorists and military personnel to carry out the infamous killings.
Three members of the popular band were taken from their tour bus and shot dead on a country road after a gig in Banbridge, County Down in July 1975.
They were travelling home to Dublin when a fake army patrol made up of Ulster Defence Regiment soldiers and Ulster Volunteer Force 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 commander Robin 'The Jackal' Jackson 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, the legal action is being taken by Mr McAlea, Mr Travers, and the families of Fran O'Toole and Brian McCoy.
Their solicitor, Michael Flanigan, confirmed that writs have been issued against both the MoD and Chief Constable.
He said: "These actions are based primarily on the fact that the Ministry of Defence is responsible in law for the actions of their soldiers.
"But it goes much further than that; documents unearthed by the likes of Justice for the Forgotten and the Pat Finuncane Centre show that the British Army knew there was a problem with loyalist subversion in the UDR for years before the attack on the Miami Showband and did nothing about it."
Mr Flanigan added: "The proceedings will also examine the role of Special Branch in vetting membership applications to the UDR and the use of agents such as Robin Jackson."
With statements of claim served on both defendants, legal attempts will now be made to gain access to all relevant documents.
It is expected to be next year before the case is heard in the High Court.