By Mark Simpson BBC Ireland correspondent

Even by Northern Ireland's standards, the latest developments at Stormont have been extraordinary.

Parliament Buildings - the seat of power in Belfast - suddenly contained more police officers than politicians as a major search was carried out of a Sinn Fein office.

It is worth remembering that Sinn Fein are not just an ordinary party.

They have two ministers governing Northern Ireland - Martin McGuinness is in charge of the province's education system, while Bairbre de Brun oversees the department of health.

What happened at Stormont is the equivalent of police storming a Labour Party office at Westminster.

The search lasted less than an hour, but the repercussions could be felt for a very long time.

'Hanging by a thread'

It wasn't just Stormont which was searched, but a number of addresses in west and north Belfast.

It was all part of a major police investigation into allegations that the IRA had a 'mole' within the Northern Ireland Office - with access to the correspondence of the Northern Ireland Secretary John Reid.

Police are investigating a 'mole'

Dr Reid's office is at Castle Buildings, less than a mile from Parliament Buildings. The Good Friday Agreement was signed at Castle Buildings four years ago.

Now, alleged IRA intelligence-gathering in that very building could damage the Agreement - perhaps fatally.

The Assembly is the public face of the Agreement, and it is hanging by a thread.

It seems to be a question of "when" not "if" the power-sharing executive is going to collapse. If the prime minister doesn't decide to suspend the Assembly, then unionists are likely to walk away.

Paramilitaries

The timing of the walk-out could be the subject of some wrangling within the Ulster Unionist Party.

Leader David Trimble is not yet ready to resign as First Minister, but the anti-Agreement MP Jeffrey Donaldson is demanding immediate action.

David Trimble: May face calls to resign

If Ulster Unionist ministers are still in the executive by the end of this month, Mr Donaldson is likely to call another meeting of the party's ruling council to force his leader's hand.

It may not come to that. Mr Trimble may decide after talks with the prime minister in the coming days to take matters into his own hands.

In spite of the peace process, the security situation in Northern Ireland is getting worse rather than better.

According to police, all the main republican and loyalist paramilitary groups have been active in recent weeks.

'Yellow card'

This is one of the reasons why confidence in the Good Friday Agreement is draining away, particularly within the unionist community.

In July, the government warned that any further paramilitary activity would not be tolerated. It was, in a sense, a political 'yellow card'.

In spite of the latest alleged IRA activity, it would be a surprise if Dr John Reid produced a red card, and declared Sinn Fein unfit for public office.

At this stage, no-one has been convicted of anything, and the government will be reluctant to rush to judgment.

But Dr Reid knows that many unionists have already made their minds up, and are no longer prepared to share power with Sinn Fein.

Devolution, it seems, is heading for a fall.

The government's role in the coming weeks may simply be to ensure the political process receives a soft-landing.