Previously unseen British army intelligence documents have linked undercover British soldier Robert Nairac to the Miami Showband Massacre.
Three members of the band, including lead singer Fran O’Toole, died when loyalist killers stopped their minibus at a bogus UDR check point near Banbridge in Co Down in July 1975.
The attack was carried out by members the Glenanne Gang, which included RUC, UDR and UVF personnel.
Two loyalists also died when the bomb they were planting exploded prematurely.
British army documents have now linked SAS trained officer Robert Nairac to the atrocity.
While he has previously been connected to loyalist murders this is believed to be the first time MoD documents naming him have been made public.
Captain Nairac was abducted and killed by the IRA in 1977 and his body has never been found.
He is one of three people belonging to the group known as The Disappeared whose remains have yet to be located.
Former RUC officer: No evidence that Captain Robert Nairac was involved in murder of IRA member
The Ministry of Defence papers were recently disclosed to solicitor Michael Flanigan who represents Fran O’Toole’s widow Valerie Andersen.
She is taking legal action against the MoD and PSNI chief constable.
It is understood the redacted documents contain suggestions that Captain Nairac obtained equipment and uniforms for the killers.
The file also claims that the British solider was responsible for the planning and execution of the attack.
Survivors, including justice campaigner Stephen Travers, have previously insisted a member of the killer gang spoke with an English accent.
In his 2015 book about the life of Captain Nairac, Alistair Kerr claimed the British soldier went on leave to Scotland on the same day as the Miami massacre.
Read More: MoD says it's 'inconceivable' Captain Nairac was at Kingsmill massacre
Mr Travers last night said that when he learned of the document it was a “huge disappointment to me that I was right.
“It was the British army involved in the planning an execution,” he said.
It is believed many of the documents provided to Mr Flanigan have been redacted and that public interest immunity certificates have also been issued.
A hearing linked to the case is due to be heard in Belfast this morning.
Mr Flanigan last night said collusion was a feature.
"This is a case where collusion is self-evident and in those circumstances it is of concern that the defendants are seeking to rely so heavily on Public Interest immunity,” he said.
“We feel the state should be as open as possible in a case of this nature and will be asking the court to look at this issue."
