An IRA mole in the Irish Republic's police tipped off a terrorist hit squad who murdered two of the most senior RUC men to die in the Troubles, a damning collusion report has found.
Chief Superintendent Harry Breen and Superintendent Bob Buchanan were gunned down on March 20 1989 near the border in south Armagh shortly after a meeting with gardai in Dundalk, Co Louth.
An eight-year tribunal found the officers might have been saved if the Garda had passed intelligence to the RUC that Mr Buchanan was on an IRA kill list.
Inquiry chief Judge Peter Smithwick levelled a blistering attack on the Garda, accusing the force of putting its reputation above the truth.
The 500-page report is a damning expose of collusion, bad policing and misguided loyalty in the Garda.
As well as confirming long-held suspicions of the IRA mole in Dundalk, the judge accused Garda chiefs of trying to undermine a retired superintendent who testified that he passed intelligence on the Buchanan death threat to the highest ranks.
"The integrity of and confidence in An Garda Siochana can properly be maintained only if suggestions of inappropriate or illegal conduct by members are taken seriously, transparently and thoroughly investigated and, above all, not tolerated or ignored on the basis of some misguided sense of loyalty to the force or to its members," he said.
William Buchanan, son of the murdered RUC officer, said it was incredible and shocking to think his father had effectively been set up to be murdered.
The Breen family said their respected RUC husband and father had been betrayed by his police counterparts in the Republic.
Tanaiste Eamon Gilmore, Ireland's Foreign Affairs Minister, apologised to the families and said the country must confront the grave findings.
"I am appalled and saddened by this finding. It is a matter of grave public concern," he said.
Unionist politicians said the tribunal will have major implications for how the past is dealt with while Northern Ireland Secretary Theresa Villiers said she would raise the findings with the Irish Government.
The tribunal found an unidentified IRA mole leaked information about the movements of Mr Breen and Mr Buchanan on the day they were killed.
No direct evidence of collusion was uncovered.
The findings are based on "live" intelligence from PSNI Assistant Chief Constable Drew Harris, and Garda intelligence reports in the years after the killings.
The report found evidence that former Garda detective sergeant Owen Corrigan passed information to the IRA - but it could not be proved he colluded in the murders.
It was further concluded that retired Garda sergeant Leo Colton was obviously trusted by the Provos - he helped them get false passports - but there was no evidence of a relationship between them at the time of the killings.
Judge Smithwick also said Mr Colton's colleague, former Garda sergeant Finbarr Hickey, who signed off on the forged passports, was foolish and naive but was not a member of or sympathiser with the IRA.
The judge holds up former border superintendent Tom Curran, who regarded himself as a friend of Mr Buchanan, as a man of integrity for revealing that he passed on intelligence on the Buchanan death threat and suspicions about Mr Corrigan's links to IRA figures.
Files on the death threat could not be found at Garda headquarters in Dublin, the tribunal found.
Judge Smithwick hit out at a lack of accountability among authorities in Ireland.
"The culture of failing adequately to address suggestions of wrongdoing, either for reasons of political expediency or by virtue of misguided loyalty, has been a feature of life in this state," he said.
The RUC and Garda are both criticised for rushing to rule out suspicions of collusion in the days following the murders.
"This was an example of the prioritisation of political expediency in the short term, without due regard to the rights of victims and the importance of placing justice at the centre of any policing system," the judge said.
Eugene Crowley, a former commissioner who died in 2009, faces unanswered allegations that he ignored information from Mr Curran about a Garda's alleged links to the IRA - Owen Corrigan.
"It is and will remain a mystery why Eugene Crowley reacted as he did," the judge said.
Garda Commissioner Martin Callinan said it would be inappropriate to comment without fully examining the report.
The Police Service of Northern Ireland said the double murder is an open investigation and it will fully consider any new evidence.
Mr Breen and Mr Buchanan travelled to Dundalk to discuss a possible joint RUC/Garda operation on lands owned by prominent republican Thomas "Slab" Murphy.
Mr Breen also claimed at the meeting that Mr Corrigan was receiving payments from Murphy.
The IRA are thought to have ben tipped off at about 11.30 on the morning of the ambush by the IRA mole in Dundalk station.
But Judge Smithwick has not ruled out a second act of collusion by the same, or possibly a different, member of the Garda that day. He said it is likely a mole confirmed the officers had arrived in the station.
On their way home on the Edenappa Road near Jonesborough, just a few hundred yards over the border, a van with a gang of up to six armed IRA men cut them off.
Both policemen were in civilian clothes and neither was carrying a weapon.
Mr Buchanan tried to reverse the red Vauxhall Cavalier out of danger but got stuck in a ditch and was shot several times in the driver's seat.
Mr Breen tried to surrender. He got out of the car, walked to the front of it with his hands up and was shot several times.
The tribunal was established in 2005, with lawyers spending six years examining intelligence and witness statements from police, undercover agents, Provo bombers and politicians during 133 days of public hearings.
It was set up as part of a peace process deal after Canadian judge Peter Cory recommended a public inquiry be held into allegations of collusion by garda officers, or a civilian in the force, over the murders.
The Superintendents' Association of Northern Ireland said they would carefully consider the report.
A statement said: " This will be a difficult and emotional time and whilst we carefully consider the findings of the report, we must be sensitive to the needs and the feelings of the families. Those considerations must remain paramount.
"Without doubt, the conclusions of this report will make stark and challenging reading for many people and whilst we recognise this step towards bringing out the truth in relation to these tragic and horrendous murders, what is now important for us is to see how these findings are acted upon."