Four people, one of them a party official in the Northern Ireland parliament, were arrested in dawn raids aimed at alleged intelligence-gathering by the IRA, police officials said.
"A number of items [were] seized," a police spokesman added, as it emerged that those arrested included a Sinn Fein parliamentary administrator and a former messenger at the Northern Ireland Office.
Police officials said the raids were linked to IRA intelligence gathering, though not to a break-in and theft of Special Branch files at Castlereagh in Belfast in March 2001.
The head of Sinn Fein's administration team at Stormont, Mr Denis Donaldson, was one of those taken in for questioning.
A party spokesman told ireland.comup to five homes of Sinn Féin members and community activists in west Belfast were raided at 5 a.m.
"Several persons were arrested and a number of items seized," a police spokesman said, adding a search of the Stormont offices of an unidentified individual were continuing. He said documents related to policing, human rights and justice issues were taken.
Ms Bairbre de Brún, a minister in the executive, said: "One of the things we are very anxious to learn at this point is whether John Reid had a part in this and if he signed this warrant or authorised it, it is something he needs to explain".
Northern Ireland Secretary Dr John Reid defended the actions of the Police Service of Northern Ireland, saying "the police will follow wherever the evidence leads them".
In response to calls for sanctions against Sinn Fein, Dr Reid said there was "a judicial process going on and if charges are brought, then I certainly will not pre-empt that decision, which has to be taken after investigation and after charging. "On the basis of the evidence, decisions will be reached and then we will consider that, but it would be quite wrong to pre-empt any judicial process." "I have been aware for a considerable time that this investigation was ongoing. I was aware of it prior to July 24th. The knowledge of that investigation was part of the reason why I made the statement on July 24, warning people - as I did again yesterday - that democracy and any involvement in terrorist activity were completely incompatible." "You can't ride two horses, particularly when those horses are so far apart as terrorism and democracy." Ms de Brún said the search was a political attack on a political party. "It is absolutely outrageous and it is clearly part of a political picture of intervention by the police service of Northern Ireland.
"It is stemming from the time in the spring when David Trimble indicated he could foresee the possibility of bringing an end to the political institutions".
It emerged tonight that senior civil servants and police chiefs fear confidential letters to ministers may have been photocopied and handed to the IRA.
Police refused to comment, but according to authoritative security sources in Belfast tonight, this is one line of inquiry being followed by detectives questioning three men and one woman.
Sources also said that criminal charges would follow.