Sinn Fein has been challenged to come clean and admit that the IRA was responsible for murdering 10 innocent Protestants in the Kingsmill massacre.
As the 40th anniversary of the atrocity approaches next week, Ulster Unionist MLA Danny Kennedy called on the party to finally tell the truth about the south Armagh attack.
"It's time for Sinn Fein to stop the lying and admit that the IRA carried out this bloody sectarian slaughter of innocents," he said. "That is the very least they owe the victims."
Mr Kennedy issued his challenge as a Catholic man, whose three brothers were gunned down in a loyalist atrocity the day before Kingsmill, appealed to the Rev Ian Paisley's son Kyle to retract a false allegation that his late father once made linking him to the IRA.
Eugene Reavey from Whitecross in south Armagh said the former DUP leader had done him "a grave injustice" and he hoped his son would rectify it.
Next week marks the 40th anniversary of one of the bloodiest periods in the Troubles in which 16 people were murdered in three shootings within 24 hours.
The UVF's infamous Glenanne gang shot the three Reavey brothers in Whitecross and three members of the O'Dowd family in Gilford, Co Down, on January 4. The next day, the IRA killed 10 Protestants in Kingsmill.
The Provisionals were officially on ceasefire when a 12-man gang stopped the workers' minibus, ordered the men out and then opened fire.
The attack was claimed by the South Armagh Republican Action Force in retaliation for the Whitecross and Gilford attacks.
A 2011 Historical Enquires Team report dismissed that claim of responsibility, and blamed the IRA.
But Sinn Fein's Mitchel McLaughlin responded by saying that the party believed "the denials by the IRA that they were involved".
Last night, Mr Kennedy, who knew nine of the dead, said: "Sinn Fein's ongoing denial lacks all credibility and is an insult to Kingsmill survivor Alan Black, and to the bereaved.
"It is hypocrisy for the party to demand the truth from the authorities over shootings while continuing to lie about the IRA's actions.
"As the 40th anniversary approaches, Sinn Fein must at last come clean."
The Kingsmill families will hold a memorial service on Tuesday morning in the village, which will be addressed by Bessbrook pastor Barrie Halliday and loyalist victims' campaigner Willie Frazer.
Mr Frazer said he was hopeful that one of the gunmen, who is now living in the Republic, would be arrested and charged by gardai this year.
PSNI and Police Ombudsman investigations into massacre are ongoing and a fresh inquest is due to be held.
"No one has ever been charged, but we believe there is enough evidence for some of the perpetrators to face justice," Mr Frazer said.
Eugene Reavey, whose three brothers were shot dead by the UVF in Whitecross, said he expected the Police Ombudsman to soon make arrests in connection with the triple murder.
In 1999, the Rev Ian Paisley used parliamentary privilege to accuse Eugene Reavey of involvement in Kingsmill. He said his information came from an RUC document, but the police said no such document existed and Mr Reavey was innocent, an assertion supported by Alan Black.
Mr Reavey said: "Ian Paisley compromised and made peace with old enemies in his last years. But he never did the decent thing and withdrew his slur against me. I wrote to him at least 10 times, and to Eileen too, but they didn't reply."
The south Armagh man also told how he was now planning to ask Kyle Paisley to retract his father's remarks.
"I've watched Kyle commentating on politics and he seems a very reflective, just and principled person," he said.
"I'll be writing to him in good faith and I hope he will have the decency to respond."
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