Relatives of people killed in the McGurk's Bar atrocity are launching legal action to force disclosure from authorities.
Fifteen people were killed when the UVF detonated a bomb in the North Queen Street bar in north Belfast in December 1971.
At the time security forces blamed the IRA but this was later shown not to be true.
Campaigners believe there was collusion in the murders and that attempts were subsequently made to mislead the public.
A stepson of Thomas McLaughlin is now taking legal action against the Ministry of Defence, PSNI and Police Service and Secretary of State.
In recent years research charity Paper Trail has uncovered new evidence relevant to the case.
Relatives subsequently brought civil proceedings against the PSNI, MoD and Secretary of State, which lawyers say is now at an advanced stage.
They also say these proceedings have been delayed through continued failure to disclose documentation to their lawyers.
Eoin Murphy, of Ó Muirigh Solicitors, said "there have long been suspicions of security force involvement in this atrocity.
"This is reinforced by the catalogue of documentation which has been uncovered to date," he said.
"The families know that there are undoubtedly hundreds if not thousands of discoverable documents currently retained by the defendants which are linked to the murders of their loved ones."
Mr Murphy added that there is "an obligation on the defendants to provide these documents at the close of pleadings but thus far have failed to do so.
"It is regrettable that the only way to achieve this important and crucial disclosure for families is through the continued intervention of the High Court,” he said.
A spokesman for the British Government: "We don't comment on ongoing legal proceedings."
