In the passenger seat, Chief Supt Harry Breen was wary. He'd been uneasy since the meeting with gardai was arranged.

He'd told colleagues he "didn't like the setup", believing at least one guard, whom he named, worked for the IRA.

Breen had been sent to the meeting to draw up a joint strategy with gardai to end the multi-million cross-Border smuggling empire of senior IRA commander, and later chief-of-staff, Thomas 'Slab' Murphy.

The order was that Slab "must be shut down". It was a goal to which Breen was deeply committed so, despite his misgivings, he attended the meeting. He was due to meet customs and excise officials the next day.

The most high-risk area for the RUC men on their journey home was nearest the border. The 'Gap of the North' was long hostile to agents of the British state. In the 17th century, Irish rebels ambushed English soldiers there, then beheaded them.

Their bodies were sent back to their superiors strapped to horses.

As Breen and Buchanan drove along the Edenappa Road, near the village of Jonesborough on 20 March 1989, a sixstrong IRA team was waiting. Their car was riddled with bullets. Within seconds, both men were dead. They were the most senior RUC officers killed in the Troubles.

"Every time I pass the spot where my two colleagues were murdered I think, 'there but for the grace of God go I', " says ex-RUC officer Andrew Hanna*. Stationed in Bessbrook RUC station, Hanna collated intelligence on the IRA in south Armagh. "I regularly went to meetings in Dundalk. Had I not been on holiday that week, I'd probably have been at that meeting.

"I have a duty to my dead colleagues. I have evidence which challenges the established wisdom about what happened, and strengthens the case that they were killed as a result of IRA/gardai collusion."

Collusion allegations in the double murder have previously been dismissed by political and security figures on both sides of the border. However, two years ago the government set up a tribunal, chaired by retired judge Peter Smithwick, to conclusively address the claims.

Full public hearings should begin early next year. Dozens of people have already been interviewed. Hanna is willing to speak to tribunal investigators. "I'm challenging key 'facts' of the Breen-Buchanan murder: that the Dundalk meeting was arranged only a few hours in advance; that gardai had no earlier knowledge of it so a substantial warning couldn't have been given to the IRA; and that the IRA simply saw my colleagues' car in Dundalk and put their operation together in an hour."

Hanna says the Dundalk meeting was actually arranged three days in advance, allegedly giving the IRA ample time to plan the attack after a tip-off from a garda informer.

All RUC officers were 'legitimate targets' in IRA eyes. However, the organisation singled out Harry Breen's role in the SAS killing of eight IRA members in Loughgall, Co Armagh, two years earlier.

Hanna questions this: "Harry was interviewed on TV after Loughgall, but he wasn't centrally involved in its planning.

I believe the main motivation for killing him was to stop imminent action against Slab Murphy.

"The order had come from the top of the RUC that Slab had got away with too much for too long. He was making so much money, he had to be shut down.

Harry and Bob were to coordinate the operation with gardai. Harry had a huge amount of information on Slab. He didn't log it all, a lot was kept in his head."

IRA search After killing Breen and Buchanan, the IRA searched their car, taking confidential documents which they claimed related to "cross-border collaboration". Interestingly, the IRA didn't disclose even a vague outline of this material. The Sunday Tribune has been told the documents concerned Slab Murphy.

With 32 years' service, Chief Supt Harry Breen (51) ran H Division, which covered counties Armagh and Down. Former RUC sergeant Jack Weir later accused him of collaborating with loyalist paramilitaries and supplying weapons to the UVF. Breen was hated by republicans.

Andrew Hanna views him differently:

"It's untrue that he colluded with loyalists.

Harry Breen was a gentleman, he was old school. He carried a dress handkerchief in his suit pocket." Breen rarely crossed the border to meet gardai.

By contrast, the 20 March meeting was Bob Buchanan's 25th trip south that year. Buchanan (55) was Border Superintendent, liaising with gardai on crime and paramilitary activity.

"Bob was very religious, " says Hanna.

"Once, when we visited Drogheda garda station, the divisional commander produced a bottle of whiskey and said 'Would you lads like a wee drink?' Bob looked horrified and said, 'No, we would not.' I was thinking, 'What do you mean we, speak for yourself, Bob!'" Buchanan was "a nice guy but not half the policeman Breen was, " Hanna says.

"Bob was too easygoing. He shirked basic security. He nearly always took the same route to and from the south, and he drove the same car for three years. It had no armoured plating or bullet proof glass.

"Regulations prevented us bringing firearms into the south. Harry and Bob obeyed, I didn't. The only way they'd find out I'd my gun would be if we were attacked, and then it wouldn't matter . . . I'd either be dead, or alive because I'd used it."

The British government appointed Canadian judge Peter Cory to investigate alleged collusion in the Breen-Buchanan murders. In his report, Cory says the fatal meeting of 20 March 1989 in Dundalk garda station was "informal, unscheduled and arranged by telephone that morning".

Hanna insists the meeting was arranged three days earlier. "On Friday 17 March, I was called to an urgent meeting in Armagh RUC station. It was very highgrade, not run-of-the-mill. There were senior figures there from Special Branch and the British Army. Assistant Chief Constable Archie Hayes chaired the meeting.

"Bob Buchanan was present. Harry Breen wasn't . . . he was off that week.

About 99% of the meeting focused on Slab Murphy. Assistant Chief Constable Hayes told Bob Buchanan to phone Dundalk to arrange to meet gardai on Monday with Harry Breen. Bob then left the room to make that call."

Harry Breen was also contacted.

Sources say the day before the Dundalk meeting, he told another RUC officer, whose name is known to the Sunday Tribune, that he was unhappy with the planned visit. IRA strength in South Armagh meant it was too dangerous for the RUC to have British Army accompaniment.

Repeated concern Just before he left for Dundalk, he repeated his concern to his staff officer, Sgt Alan Mains. He named a Dundalk guard whom he believed was passing information to the IRA. Eoin Corrigan, a Special Branch sergeant in Dundalk, has since been named by DUP MP, Jeffrey Donaldson, in the House of Commons as the guard whose tip-off led to the killings.

Corrigan, now retired, has described Donaldson's allegation as "a monstrous lie". In an interview with the Dundalk Examiner, he said: "Nobody was more opposed to the IRA than me. I'd 30 years of service, in and out of the Special Criminal Court, and keeping surveillance on subversives at great personal risk." He said he'd been falsely accused possibly because of friction with colleagues in Dundalk.

An IRA informer also alleged garda collusion in the Breen-Buchanan murders. A source told the Sunday Tribune that this informer claimed he and M, a senior IRA figure, travelled to Dundalk on 20 March 1989. While Breen and Buchanan were inside the garda station, a guard left the building and told the two IRA men that the RUC officers would be leaving shortly.

There have been widespread claims in republican and security circles that M, who is now involved with republican dissidents, is a British agent. If that is so, and if he was working for the British in 1989, the possibility arises that he told his bosses of the plan to kill Breen and Buchanan and they did nothing.

The Dundalk meeting began at 2.10pm and ended around 3.15pm. The IRA didn't know which of four possible return routes Breen and Buchanan would use, so it posted teams on all four, involving possibly 20 members. Another dozen would have been involved in support roles, such as providing safe houses.

The large number of men involved strengthens the theory that the IRA had substantial prior warning. However, it isn't totally impossible that, knowing RUC officers visited Dundalk regularly, the IRA had planned a general operation to assassinate them, then fine-tuned it at short notice.

Just moments before the RUC men's car appeared on the Edenappa Road, two armed men in army combat gear, with camouflage paint on their faces, stopped three southern-bound vehicles, ordering the occupants to lie on the roadside.

They then parked the vehicles to allow room for only one car to proceed, and slowly at that, on the road. As Breen and Buchanan passed the two IRA men . . .

probably mistaking them for British soldiers . . . a cream van following behind overtook and blocked them in.

Four gunmen in combat gear and balaclavas jumped out and fired into the RUC men's car. Buchanan tried to reverse but the vehicle stalled. He died strapped in his seat. Although hit, Breen managed to get out. He waved his white dress handkerchief in the air. One gunman walked over and shot him in the back of the head.

The Sunday Tribune has been told that a witness at the Smithwick inquiry may name one of the gunmen as a Sinn Fein politician. A Kleenex tissue and a Lucozade bottle were found at the murder scene, but neither fingerprints nor saliva was obtained from forensic tests.

DNA science has since progressed and if these items have been retained they could prove significant.

The white van was located by helicopter two days later. But security precautions meant it was a week before the British Army could move in. By then, the vehicle had been burnt.

Andrew Hanna, who has since left the police, is just one of several ex-officers with information for Smithwick. Willie Frazer of IRA victims' group, Families Acting for Innocent Relatives (FAIR), has written to the tribunal on their behalf.

"These officers want to be interviewed but won't travel to the south. I've told Smithwick they're available for interview in the north, but so far nobody has bothered to come to talk to them. It hardly promotes confidence in the tribunal."

On the anniversary of the BreenBuchanan killings, Hanna lays a wreath at the murder scene. "The best we can do is flowers. We can't erect a permanent memorial. It's not that it would be destroyed . . . it couldn't even be built safely to be destroyed."

*The officer's name has been changed to protect his identity.