Sam Kinkaid, Northern Ireland’s most senior police detective, read carefully from a typed sheet to the group of bereaved relatives gathered around the boardroom of Omagh library last Wednesday.

“I was sitting directly opposite, looking him in the eye, and I could hardly believe it,” said Michael Gallagher, whose son Aidan was one of 29 people (including a woman pregnant with twins) murdered in the August 1998 Real IRA bombing. Kinkaid was saying that gardai and MI5 had withheld intelligence from two informers.

One of the sources was Dave Rupert, an American trucker who infiltrated the Real and Continuity IRAs for MI5 and the FBI. The second source was Paddy Dixon, a crooked motor dealer who supplied stolen cars to terrorists and kept the gardai informed.

Kinkaid, an assistant chief constable, retires tomorrow. In his final months of service he has delivered a series of shocks to the political system. It was he who pinned the Northern Bank robbery on the Provisional IRA; he who revealed to the Northern Ireland policing board that IRA decommissioning was incomplete. Sinn Fein branded him an old-style securocrat, but this time it is the security establishment, north and south, that will be embarrassed by his claims.

Yet this is no solo run. Peter Sheridan, Kinkaid’s successor as assistant chief constable, sat beside him in Omagh library last week, nodding in agreement. So did Superintendent Norman Baxter, who heads the Omagh investigation on a day-to-day basis, and Colin Monteith, his No 2. All three agreed that MI5 had known five months in advance of a plot to bomb either Omagh or Londonderry with a Vauxhall Cavalier car, and knew that one of the suspects lived in Omagh.

They passed on details of the plot to gardai, but never told the RUC, as the Northern Ireland police force was then known. Meanwhile, the gardai knew from Dixon that a car had been stolen for an attack on Northern Ireland, but had not intervened for fear of blowing his cover.

The result, as Sheridan told the grieving relatives, was that both Omagh and Derry were on a low state of alert when the bombers struck in August, using a Vauxhall Cavalier. An anonymous telephone warning on August 4 saying a gun and rocket attack on Omagh was planned for August 15 was discounted as a crank call by Special Branch. Even after the attack, the gardai and MI5 withheld the information.

Stanley McComb, whose wife Ann died in the bombing, said: “We are trying to get on with our lives and something like that brings it all back and it makes us frustrated, mad . . .

“We want to meet Eliza Manningham-Buller, the head of MI5, and Michael McDowell, the Irish justice minister, and we want straight answers.”

THE evidence behind Kinkaid’s claims comes from two main sources: e-mails sent by Rupert to his MI5 handlers while he worked in Ireland between 1996 and 2001, and notes kept by John White, a garda detective who handled Dixon under the direction of Detective Chief Superintendent Dermot Jennings.

Rupert, from upstate New York, had moved to Chicago, where he was mixing with the hardline Irish Freedom Committee (IFC) when an FBI agent, Ed Buckley, recruited him in about 1995.

Rupert’s business was failing, so his motive for co-operating with the FBI was at first financial. According to Lou Stephens, a financial investigator who formerly headed Irish operations in the FBI, Rupert had been “heavily financed and probably defaulted”.

Rupert headed for Bundoran, Co Donegal, where he befriended Joe O’Neill, a veteran republican who owned a pub in the town. Later Rupert rented a bar of his own, the Drowse Inn, in Leitrim, which he loaned to the Continuity IRA for meetings. It is thought the premises were bugged.

Times Recommends

• Should Knox’s trial even have reached the courtroom?
• 'Was it real?' The US bailout nightmare
• Iranian opposition plays the long game