Senior IRA sources have admitted that a man who previously worked in Castlereagh police station had republican connections. However, they have continued to deny the paramilitary group was behind last month's break-in at the base.
The man, who has long-standing republican sympathies, slipped through vetting procedures to secure a job as a civilian worker at Castlereagh.
It is understood he became involved in Irish-American politics in the early 1990s.
He moved from the US to Belfast several years ago, according to security and republican sources on both sides of the Atlantic. It is understood that, on arrival in the North, he worked in a Belfast restaurant. He was then employed in Antrim Road police station in north Belfast before moving to Castlereagh.
It is understood the Provisional IRA has received high-quality information on the workings of the base and on Special Branch and other police officers.
While acknowledging the man's connections, republican sources denied all knowledge of the St Patrick's night raid on the base during which highly sensitive documents were stolen.
"We are not going to be scapegoated for that," they said.
Earlier this week in a statement to the BBC, a Provisional IRA spokesman insisted the paramilitary group was not involved.
Sinn Féin president Mr Gerry Adams said he has no knowledge of IRA involvement: "I have no evidence that there was any republican involvement in the business at Castlereagh."
However, senior police sources last night insisted the Provisional IRA was behind the raid. "We are now following one line of inquiry only and that is one of IRA involvement," a source said.
It is understood that within 24 hours of the raid a senior police officer told the North's First Minister, Mr David Trimble, the IRA was responsible.
Sources said the Police Service of Northern Ireland (PSNI) was now aware of the key players involved in the raid. Detectives travelled to New York last week to question a former employee at Castlereagh in connection with the incident. It is understood the man, who had recently left his job as a chef, voluntarily gave himself up to the FBI.
"Our team has returned to Belfast but our interest in the chef is undiminished," a senior police source said last night. So far the PSNI has arrested nine people in connection with the raid. Eight were released without charge.
One man was charged with offences not related to the Castlereagh break-in. Security sources have acknowledged some of those questioned have since been eliminated from police inquiries.
Further arrests are expected. The stolen documents contained the telephone numbers and code names of Special Branch informers and the names and home telephone numbers of around 200 officers.
Ulster Unionist MP Mr Jeffrey Donaldson said he remained "very worried" about the risk to security force personnel.
He expressed concern that as evidence of IRA involvement increased, attempts would be made at a "cover-up" to save the Sinn Féin leadership from political embarrassment. "That must not happen," he said.