By Brian Rowan BBC Northern Ireland security editor

The pictures from the police raid at Stormont on 4 October were full of drama.

That was the morning officers arrived in force at the home of Northern Ireland's power-sharing government to search a Sinn Fein office.

The arrests and searches happened in early October, but this operation had been in the making long before then

We were told it was part of an investigation into IRA intelligence gathering, but nothing of an incriminating nature was found.

The police went to Parliament Buildings after arresting Sinn Fein's head of administration at the now suspended Stormont Assembly.

He is one of four people charged as part of Operation Torsion which has uncovered IRA intelligence gathering at the heart of government in Northern Ireland.

Police leave after raiding Sinn Fein office at Stormont

The arrests and searches happened in early October, but this operation had been in the making long before then.

An impression was allowed to develop that the police investigation came after a former Northern Ireland Office employee was caught photocopying an Army document some 14 months ago, but that is not the case.

The former messenger at Castle Buildings - the headquarters of the Northern Ireland Office - was spotted by a police officer, one of the then Secretary of State John Reid's bodyguards.

The matter was dealt with internally, within the Civil Service, and was not reported to the police at that time.

In fact, the police lead came after the Special Branch robbery at Castlereagh in March this year and the information which triggered Operation Torsion was provided by one of their sources deep within the IRA itself.

Bugging

"This was brought about by the raid on Castlereagh," a security source told me. "It would never have happened if Castlereagh had not happened."

By the summer, John Reid had authorised a major bugging and surveillance operation targeting the IRA.

As part of this, Special Branch sought and was given assistance by the UK's national intelligence service MI5.

It was around the same time - on 24 July to be precise - that Dr Reid gave the IRA a final warning: All activities, including targeting, had to cease.

John Reid: Authorised bugging operation

I understand Prime Minister Tony Blair was briefed by John Reid in September, by which time the investigation had reached a critical phase.

On the police side, only a handful of senior officers were kept informed - the then Acting Chief Constable, Colin Cramphorn, senior Special Branch officers Chris Albiston and Bill Lowry, and the senior Belfast detective Phil Wright, who is also heading the Castlereagh break-in investigation.

Before Operation Torsion "went live", Chief Constable Hugh Orde had been fully briefed and the most senior uniformed officer in Belfast, Alan McQuillan, had been brought into the picture.

Arrests

The operation spanned a period of months and, according to one source, the main target was the IRA's so-called director of intelligence - a prominent west Belfast republican.

But he managed to avoid the net.

Four arrests were eventually made on 4 October and three men and a woman were subsequently charged. In the search operation, hundreds of documents and a lap top computer were seized.

But, I understand that long before this date, the police were already aware that the IRA had obtained copies of sensitive government documents as well as details on prison officers - the latter may have been copied from a human resources computer database at Prisons Control.

'Big catch' sought

An assessment was made that the prison officers were under no threat from the IRA and Operation Torsion was allowed to run.

The police were looking for a bigger catch.

All the sensitive information recovered during Operation Torsion may still be in the hands of the IRA - sources believe the organisation may have copied it onto a computer system across the border in the Republic of Ireland.

The damage done to the IRA's intelligence gathering machine is still being assessed.

But what is certain is that much damage has been caused to the political process and Tony Blair believes the only way power-sharing will be restored is if the IRA ceases all activities and steps off the stage.

He wants the IRA to disband, but it has already dismissed that as an un-realisable demand.