The Northern Bank was immensely proud of its headquarters in Belfast’s Donegall Place. Built in the 1970s to withstand the worst of Northern Ireland’s terrorist troubles, it has at its heart a vast concrete bunker protected by heavy steel doors and a state-of-the-art security system.

So taken was the bank with its impregnability that occasionally its favoured clients, after enjoying a hearty lunch in the boardroom, were treated to a tour of the vaults. Bank executives pointed out the elaborate security as they descended the concrete stairway to the windowless bunker below where employees counted and sorted millions of pounds each day.

The illusion of security was shattered late on Monday. At about 9pm the Northern Irish police received a call that would send shockwaves through the bank and now threatens to convulse Northern Ireland’s peace process. The fortress had been pillaged and nobody even noticed.

It was a spectacular coup for the robbers, and an equally spectacular embarrassment for the Northern Irish police. The once fabled intelligence gatherers who had battled IRA and loyalist terrorism for 30 years had not a clue that a big robbery was being planned. And like Harland and Wolff, the Belfast builder of the Titanic who boasted its ship was unsinkable, the top brass at the Northern Bank had not taken everything into account.

Their headquarters may have been bomb proof, but it had no defence against terror.The robbers’ plan was cruel but simple: terrorise the bank’s employees and force them to open the vaults. The plan needed expert intelligence on the inner workings of the bank’s security systems and, more importantly, the identities of key officials who would have access to the security codes and the authority to send others home.

Planning was meticulous. Last Monday was one of the bank’s busiest days of the year as it received millions from local retailers and hoarded cash to replenish the province’s cash machines. The vaults, the gang knew, would hold as much as £30m, much of it in untraceable notes.

The gang struck first on Sunday evening. Chris Ward, a 23-year-old Celtic football fan and relatively junior bank employee, was seized at his home and his family held hostage. Ward, said his friends, is a member of Erin go Brath, the Celtic supporters club, and a regular drinker at the Prisoners’ Dependents club in nationalist west Belfast, a social club frequented by republican former prisoners. The wrong people may have discovered where he worked and what he did.

While his parents, brother and brother’s girlfriend were held at gunpoint, he was driven to the home of Kevin McMullan, his supervisor, where other gang members had posed as policemen to gain access to the house.

By the time Ward arrived with his captors, McMullan’s wife, Karen, had already been gagged, hooded and driven away in her car. She had been forced to wear a white boiler suit to avoid leaving forensic traces — an early sign that the gang was “forensically aware” and determined to leave no clues for the investigation that would follow. McMullan and Ward knew the consequences of not co-operating were straightforward: they and their families would die.

The kidnappers wore boiler suits and balaclavas, the uniform of choice of the peace process paramilitaries, and were, the police believe, almost certainly members of the Provisional IRA. Publicly the police have refused to blame the IRA, saying it is only one of five organisations it believes had the capacity to plan and execute the raid.

Privately, however, they have few doubts. McMullan and Ward were seized and held in nationalist areas of Belfast where loyalists would fear to set foot. Furthermore, the planning and intelligence were of a scale that only the IRA has demonstrated in the past and the sheer daring of attempting Britain’s biggest bank robbery on Northern Ireland’s most secure bank all pointed to the Provisionals.

The next day the two men were told to go to work as normal when their shift started at 12pm. At 6pm the heist proper began. McMullan ordered the bank’s remaining security staff to go home early. It was Christmas week, so nobody complained. Once they had left, McMullan and Ward let the gang in and they quickly set to work.

The first million was loaded into a briefcase and delivered to a man in nearby Queen Street — a dummy run to check whether the two men had kept silent. An hour later, at about 7.15pm, a white van pulled into the lane behind the bank and started to load it with cash that had been prepared by Ward and McMullan. When the van was full, the gang drove off and then returned just before 8pm for a second load.

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