Police in the North are continuing to question four people arrested during raids on homes in north and west Belfast today.
The arrests were part of an operation which also involved a raid on Sinn Fein's offices at Stormont.
More than 200 officers took part in the operation, in which documents were seized.
The PSNI are reported to be investigating whether confidential letters to ministers may have been photocopied and handed to the IRA.
Sinn Fein has reacted angrily to the raids.
McDowell advises caution
The Minister for Justice, Michael McDowell, has urged people to wait for the full facts about the PSNI raid to be made clear before rushing to judgement.
Speaking on RTÉ News, Mr McDowell said he would "counsel everyone" to wait for more details to emerge.
He said details should be much clearer by next Wednesday when the Taoiseach meets with the British Prime Minister.
Mr McDowell said people should wait to see what charges are brought and the contents of the seized material before deciding why the raid was carried out and what implications it may have.
Cowen expresses concern over arrests
Responding to the raids, the Minister for Foreign Affairs, Brian Cowen, said that the events were a matter of concern for all of the parties involved in the peace process. But he added that there should be no rush to judgement.
Mr Cowen said that it was vitally important that as much information as possible was made available, as only then could a proper evaluation of the implications of the events be made.
Commenting on this morning's developments, the Northern Secretary, Dr John Reid, said he had known since before 24 July that an investigation by the PSNI had been ongoing.
However Dr Reid declined to say whether or not he had specifically known in advance about this morning's raid.
Speaking in London, Dr Reid repeated his statement made to the Labour party conference in Blackpool yesterday, that democracy and terrorist-related activity were incompatible.
The Northern Secretary said he would not be interfering in the investigative and judicial process and he denied Sinn Fein's allegations that the raids had been politically motivated.
The four people are being questioned by detectives and can be held without charge for up to 36 hours.
One of those detained is Denis Donaldson, the head of Sinn Féin's administration office at Stormont, which was also raided by armed police this morning.
Investigation into alleged intelligence-gathering
Police say the operation, which is believed had been ongoing for some considerable time, was part of an investigation into alleged intelligence-gathering by Republicans in Belfast.
It focussed on suspicions that sensitive information may have been gathered illegally at Stormont.
Another one of those arrested was a former messenger at Castle Buildings in Stormont who left the job a year ago. During his employment, he would have handled internal and external mail circulating in the offices of the Northern Secretary, John Reid, and the Stormont Security Minister, Jane Kennedy.
Security sources insist the messenger did not have access to intelligence between London and Belfast, which was transmitted on a separate electronic system.
Sinn Féin has reacted angrily to the raids, saying they were "politically motivated".
The Ulster Unionist leader, David Trimble, has called on Tony Blair and John Reid to take action against Republicans.
