One questioned over security breach

Castlereagh is the PSNI's Belfast headquarters

The police are continuing to question one man about the security breach at Belfast's police headquarters on 17 March.

Six people were arrested on Saturday in an operation involving several hundred police officers and soldiers.

Four of them were released on Sunday and another on Monday.

Security sources are now linking the incident to the Provisional IRA.

It is understood one of those arrested and released is senior republican Bobby Storey.

Detailed searches

A Special Branch officer was assaulted and documents taken when three men entered an office inside the Castlereagh complex, in the east of the city, on St Patrick's Day.

The theft occurred in the source handling unit, which is used as a "link-point" for security force informers and their police handlers.

The police investigation is now concentrating on the possibility of republican involvement, specifically the Provisional IRA, security sources have told the BBC.

Bobby Storey: Among those arrested

On Saturday, detailed searches involving the police and army were carried out at premises in west, north and east Belfast and in Londonderry.

None of those arrested, four men and two women, is connected to the security forces.

A civilian employed at Castlereagh was also being held.

One man was arrested at his home in Limewood Street in Derry during a search involving forensic officers.

Local republicans said the police searched the offices of Tar Abhaile, a group set up to support former republican prisoners.

A number of premises were raided

It is understood material was taken away from both premises.

The police said a number of people had been arrested in the Bogside area of the city.

A Sinn Fein spokesman said republicans had played "no part in the raid on Castlereagh".

He said it was an "inside job" and claimed the arrests were an "attempt by Special Branch to cover up the truth".

Anti-terrorism

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.

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.

Sir John Chilcot will be assisted by Colin Smith, a former Thames Valley chief constable and a former member of Her Majesty's Inspectorate of Constabulary.

The review will proceed in parallel with the criminal investigation which is being led by Detective Chief Superintendent Phil Wright, the most senior detective in Belfast.

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.