John Reid: Described IRA denial as "interesting"

He added: "At the end of the day, this will be based on the evidence."

The theft occurred in an office which is used as a "link-point" for security force informers and their police handlers.

Among the documents taken was an index book listing the ranks, names and telephone numbers of Special Branch officers.

However, the information taken did not include their addresses.

Details on other officers not attached to Special Branch were also listed in the book.

Special Branch officers have been briefed and told that the threat assessment following the breach was being upgraded.

But security sources said officers had not been advised to leave their homes.

Sinn Fein said a warning to Special Branch officers following the breach was an "expensive smokescreen".

'Inside job'

On Sunday, the party's spokesman on policing, Gerry Kelly, said that the officers would already have high risk security status.

Mr Kelly said he was adamant that the security breach was an "inside job".

Security sources told the BBC on Saturday there was no other major line of inquiry than that linking the IRA to the security breach.

Last week, detectives investigating the breach travelled to the United States to question a former employee at the complex.

The man police wanted to question left his job in the kitchens at Castlereagh a number of weeks before the incident.

Security sources said detectives had been liaising with the American authorities.

Two separate investigations into the incident are taking place - the police's own and an inquiry by former senior civil servant Sir John Chilcot who will report directly to Northern Ireland Secretary John Reid.

Days after the breach, it emerged that the office was only moved to that room from another part of the building a week previously because the complex was being refurbished.

Special Branch deals with intelligence work, some relating to informers, and has an anti-terrorism role in Northern Ireland.

One of the main police centres for the interrogation of terrorist suspects was located at Castlereagh. It closed at the end of 1999.