Two employees of the Northern Bank were today being questioned by detectives investigating the audacious £26.5 million raid on the bank's Belfast headquarters last Christmas.

Chris Ward, 24, and a woman aged 22, believed to be a friend, were being questioned following a fresh operation by detectives investigating Britain's biggest cash robbery, which has been widely blamed on the IRA.

Mr Ward, who lives in Poleglass, a Catholic area of west Belfast, told police that his family were held hostage by the robbery gang in advance of the heist, five days before Christmas last year.

He described how he had been taken at gunpoint to the home of a second employee, Kevin McMullan, before both were given orders to attend work as normal.

At the end of their shifts, the pair - who were both key-holders - were instructed to open the underground vault to give members of the gang access. They had been told that their families would be executed if they failed to comply.

Mr McMullan's wife Karen was blindfolded and driven to a remote forest where she was abandoned. It was only when Mrs McMullan, suffering hypothermia, reached the door of a farmhouse that the alarm was raised. By then, the gang had escaped with around £26.5 million, mostly in newly-printed notes.

Several weeks after the bank vaults were cleared, Mr Ward relived his ordeal in a television interview.

He described how he was ordered to stuff £1.2 million in cash into a sports bag and carry it from the bank in a dummy run. After a handover to one of the gang, the two employees then filled up to 24 green boxes with cash and took them to a loading bay.

At the time he said: "The media don’t say it directly but there is an insinuation that because I am a west Belfast Catholic man that I must have been part of the robbery."

Ten people have since been arrested and three people charged in connection with the robbery, but no-one has yet been charged with physically carrying out the raid.

Mr Ward, a supervisor in the cash centre, is still employed by the bank but has been on sick leave since the robbery.

At 6am today, four armoured police Land Rovers arrived at his modern end-of-terrace house. Police stood guard while forensic experts carried out a detailed search.

Neighbours in the close-knit nationalist community declined to talk about what had happened. Most were still asleep when the police carried out their raid.

One woman said: "They’re neighbours, I don’t want to say anything about them. The family keeps themselves to themselves."

Accusations over the heist, and the murder a month later of Robert McCartney, combined to place enormous pressure on the IRA to sever its criminal links which ultimately led to the group's pledge in July to down arms.

A small number of the stolen Northern Bank notes cash were seized by police across the border in Cork but the bulk has never been recovered. The Northern Bank issued redesigned currency to render the stolen money worthless.