Taoiseach Micheál Martin has said he hopes the publication of a report into the 1976 Kingsmill attack provides "some answers" to the families of those killed by the IRA in the atrocity.
Ten Protestant workmen were shot dead by the IRA in south Armagh almost 50 years ago.
The report by Northern Ireland's Police Ombudsman found that there were a number of errors in the original investigation following the attack.
In a statement, Mr Martin urged anyone with information regarding the attack to come forward "even at this late stage".
"Today, on the presentation of the report of the Police Ombudsman of Northern Ireland into the police investigation to the families of victims, I think of all those killed in the attack, and of Alan Black, the sole survivor.
"It is quite shocking that the perpetrators of this heinous crime have still not been held accountable. I call on anyone who may have knowledge of the attack, even at this late stage, to contact the authorities," he said.
Mr Martin added: "It is also a reminder of the pressing need for a comprehensive legacy framework to provide adequate mechanisms for justice and truth that meet the legitimate expectations of victims’ families."
Stormont's Justice Minister, meanwhile, cautioned against the need for a public inquiry into the Kingsmill Massacre, but was not ruling anything out.
Naomi Long said she was wary of putting families and victims "through a series of hoops" but added that a public inquiry would "of course" be pursued if other probes yielded "gaps" in understanding.
In the Stormont Assembly, Ms Long was asked by SDLP MLA Justin McNulty whether she would petition for a public inquiry into Kingsmill.
"It is not for me to call for public inquiries at this point in time," she replied.
"But I am certainly not ruling anything in or out at this early stage."
Kingsmill families 'vindicated' by Ombudsman's report
The Kingsmill families said they have been "completely vindicated" by a Police Ombudsman's report into the atrocity.
They said the authorities "walked away" from the original RUC investigation and believe it was done to protect an informer or informers amongst the IRA gang responsible.
Survivor Alan Black said the families would continue to fight for justice for his ten murdered co-workers.
A paused civil action will now recommence.
Watch: Karen Armstrong says the families are vindicated by the report
Solicitor Barry O'Donnell said the 100-page Ombudsman's report had identified a series of failings in the original investigation.
Key among them was the failure to interview a suspect who normally travelled on the works minibus, but was not there the night of the shooting.
He was later named in intelligence files as having been involved.
There were other failings around forensic opportunities
"So, you had a potential there for substantial evidence not being properly processed.
"And if you walked in the shoes of our clients, the only conclusion that you could possibly come to is that at some stage, there was an intelligence agenda being rolled out to protect those that were involved in the Kingsmill murders," Mr O'Donnell said.
Karen Armstrong, the sister of John McConville who was 20 when he was shot dead that night, said the family had struggled to rebuild their lives after her brother's murder.
"I think the point of the matter here is that the Ombudsman has totally agreed with everything that we have said in our complaint, that basically it's all in here," she said.
Ms Armstrong added: "You know that basic work, police work, wasn't completed and wasn't finished.
"Now we've been on this road for a long time, and to be honest, it's been tough, but we've had a lot of time to think what was the reason behind that?
"Why did they not even make more of an effort or to find out who was responsible for the murders?"
As well as identifying the failure to arrest and interview 11 men identified by intelligence, the Police Ombudsman also found that the original investigation failed to exploit ballistic links with other attacks in which the same weapons were used.
There were also missed investigative opportunities and inadequacies in areas such as forensics, fingerprints and palm prints, and witness enquiries.
Ombudsman Marie Anderson said the RUC investigators had been under immense pressure
"The detective leading the investigation had a team of eight to assist him in investigating ten murders and an attempted murder, which was supplemented for only a matter of weeks by two teams of about eight to ten detectives from the RUC's Regional Crime Squad. This was entirely inadequate," Ms Anderson said.
She stopped short of saying there had been collusion to protect informants.
"I have taken into account the limitations on my powers to decide on a complaint of 'collusion’ as outlined in the Court of Appeal judgment in Re Hawthorne and White.
"In consequence of the judgment of Scoffield J on 6 February 2025, I am unable to form an evaluative view of my own on conduct constituting 'collusion' or 'collusive behaviours'," she said.
However, Mr O'Donnell said the report suggests that "an intelligence agenda" was protected.
"The failings that the Ombudsman points out would lead any reasonable mind to conclude that this was an intelligence agenda being protected."
The mill workers were shot dead at Kingsmill, near the village of Bessbrook outside Newry, as they returned home from work on Monday 5 January 1976.
Mr Black was the only survivor after he pretended to be dead after he was shot.
Before the shooting started, the only Catholic on the bus was identified by the gunmen and told to leave the scene.
Mr Black complained to the Police Ombudsman 12 years ago.
He claimed several state agents among the 12-man IRA gang responsible had been protected.
Watch: Solicitor Barry O'Donnell says Ombudsman report identified failings in original investigation
Some of the families have been critical of the time taken to produce the Police Ombudsman's report.
Publication had to await the conclusion of a lengthy coroner's inquest.
A number of those thought to have been involved in the massacre are now dead.
A coroner's inquest in 2024 found that 12 members of the Provisional IRA had been responsible.
They had stopped the works bus by pretending to be an army patrol, forced the workers out, lined them up along the side of the bus and opened fire.
The killers then walked amongst the wounded shooting those still alive.
Read more: Provisional IRA lied over Kingsmill attack, says coroner
The men had been on their way home from Glennane Mill to the village of Bessbrook, where many of them lived.
Those murdered were 46-year-old Robert Walker; Joseph Lemmon, 49; Reginald Chapman 29; Walter Chapman 35; Kenneth Worton, 24; James McWhirter, 63; John Bryans 50; Robert Freeburn, 58; John McConville, 20, and 18-year-old Robert Chambers.
We'll never get justice for them. But I'm determined to get the truth.
The killings were claimed by a group styling itself the South Armagh Republican Action Force, but the murders were laid squarely at the feet of the Provisional IRA.
The organisation was supposed to be on ceasefire at the time, but had given its members wide-ranging latitude to carry out attacks.
It was reported that the massacre had been in response to a series of loyalist murders in the preceding days, but the coroner said Kingsmill would have required extensive pre-planning and had been conceived well before they happened.
The rifles used were ballistically linked to subsequent IRA shootings.
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A number of suspects were identified and several men was arrested but no one was ever charged with the killings.
In 2016, a case review identified a match from a handprint recovered from inside a van used by the gunman.
A man was arrested in Newry but prosecutors decided there was insufficient evidence to proceed.
Survivor determined to get to the truth for those who were murdered
Speaking on RTÉ's Morning Ireland, Mr Black criticised the 12-year delay in completing the report, which he said they were told was a priority.
"They’ll stand before me today and say that Kingsmill was a priority for them. But my argument to that is, how long does it take to do a non-priority case?"
He said it has been almost 50 years now since the attack and he thought that he would die before the inquiry was concluded.
"I thought, oh my God, I'm going to pass on before I get to the bottom of this because ... the coroner found there's no collusion. We know there was. We know there was an informer involved. So we're swimming against the tide ... they don't want to open a Pandora's box. And we want that box wide open."
He said that he suffers from survivor's guilt and is determined to get to the truth for those who were murdered.
He recalled the last time he saw his apprentice Robert Chambers alive, describing him as "such a happy-go-lucky young lad".
Mr Black said his last memory of Robert was him on the road "calling for his mummy - him shot and 'Mummy, Mummy, Mummy'".
"And then a gunman came over and just shot him in the head.
"So, that was my last memory of Robert, that will stay with me to the day I die, as they all will.
"And I feel duty bound ... to get some sort of truth for them. We'll never get justice for them. But I'm determined to get the truth. And if this report doesn't contain the truth, then I'm going to take it further.
"Because we do know there was collusion, we do know that there was people involved in it, even a gunman, that the police were told about him, and then they didn't even go and interview him, so we know a lot more than they think we know," added Mr Black.
