Relatives of some of those killed during the Kingsmill massacre have threatened to pull out of an inquest if legal authorities in the Republic of Ireland do not co-operate.
Colin Worton, whose brother Kenneth was among 10 textile workers gunned down during the roadside ambush, said he would not be satisfied until there was full disclosure.
He said: "The Dublin government have to step up and give all that they have because we know they have information on the killings so why not pass it over?
"We need to have the complete disclosure because we will not be happy until we have it.
"There is only one option - get the full disclosure from the south or else.
"Otherwise, it's just a partial inquest."
Mr Worton was speaking after concerns about delays in the delivery of documentation from An Garda Siochana were raised during a preliminary hearing at Belfast's Laganside Court.
Alan Kane QC said: "Justice would be denied to them (the families) if the inquest were to be completed until there is adequate disclosure of materials by the authorities in Dublin."
The factory workers were travelling home from work when their minibus was ambushed in January 1976.
They were asked their religion then lined up on a country road and shot dead in a sectarian attack blamed on the IRA. Only one man, Alan Black, survived despite being shot 18 times.
Victims' relatives are seeking details from the Garda about the weapons used, intelligence and the getaway van employed by the gunmen.
However, Garda Commissioner Noirin O'Sullivan cannot direct an officer to give evidence to the inquest without new legislation.
The court was told that a letter outlining areas of concern was sent earlier this month and that Sean Doran QC, counsel for the coroner, has met with Irish officials to discuss the case.
Judge Brian Sherrard said the material from Gardai was of "central importance" and he could not contemplate closure until all avenues had been explored.
He said: "I am mindful of the need to ensure that the totality of the information that is being sought is provided.
"This matter will not be closed until I am satisfied we have taken the appropriate steps."
Meanwhile, the court also heard that the families are to be given a document detailing the history of the weapons used at Kingsmill.
The paper, produced by the PSNI, details any links with other IRA attacks.
The inquest, which stalled to allow police to investigate a partial palm print found on a van believed to have been used by the gunmen, is expected to resume hearing evidence next month.
Judge Sherrard said he was content to proceed but cautioned that witnesses could be recalled if the Garda information threw up new leads.
"We will keep pushing with regard to this," he added.
Outside, victims campaigner Willie Frazer, who accompanied some of the families to court, said: "The Irish government has got a lot of information. They should hand it over."