Senior police and politicians have been the targets of a Real IRA intelligence-gathering operation aimed at medical files, Belfast police said.
Five people have been arrested in connection with the operation, in which the terrorist organisation potentially had access to hundreds of thousands of files giving the addresses of patients.
Dozens of people have already been warned by the police that personal details may be in the hands of the Real IRA, the organisation that killed 29 people in the Omagh bombing in 1998.
The uncovering of the Real IRA's activities is regarded as a success for the police, but also as a worrying development because the group is not on ceasefire and remains active.
One of those arrested is a man with authorised access to the computer system at the Royal Group of Hospitals, which annually treats more than a million people from all parts of Northern Ireland. The others reportedly do not work in the health service.
A sense of shock was generated by the fact that the addresses of much of Northern Ireland's population may have been available to the organisation. Security sources say police have not yet established how many people may be at risk.
Those affected by the Real IRA's intelligence operation are thought to include politicians, senior police officers and members of the Policing Board, which supervises the activities of the police.
The board's chairman, Professor Desmond Rea, is said to be one of those whose records have been sought. He described the revelation as an extremely worrying development.
The names of scores of people joining local policing committees were published recently, giving rise to the suspicion that the Real IRA had been attempting to compile lists of their addresses.
Chief Constable Hugh Orde described the Real IRA exercise as a substantial operation that had been very professionally executed.
He added: "We have five people in custody. They're currently being interviewed. We are looking at material that has been recovered to make sure that, if people are being targeted, they're being properly warned so we can look after them and their security."
The chief executive of the Royal Group, William McKee, said that the computer system was the fundamental building block for providing care, and storing information about all patients.
He added: "If these allegations are true, then we are outraged that our computer record system has been violated in this way. It breaches every standard of integrity and patient care.
"It's a dreadful kick in the teeth for the other six and a half thousand staff who work to such high, exacting standards. We are very concerned for the patients at the centre of this investigation and are deeply sorry for the distress this is causing them and their families."
Last month, republican dissidents unsuccessfully attempted to explode a device containing 1,200lb (540kg) of explosives in Londonderry. A year ago the Real IRA planted a booby-trap device that killed a worker in the same area.
The organisation has not been associated with high-profile assassinations, most of its activities taking the form of bomb attacks.
It may, however, have ambitions to widen its activities. Police warned a year ago that dissidents were planning the assassination of a senior Unionist political figure.Reuse content