Once again the mask appears to have slipped. We cannot know for sure that the Provisional IRA was behind the death of Denis Donaldson, the former high-ranking Sinn Fein official who was found shot dead in Donegal yesterday. The Republican paramilitary group has denied responsibility. But given that Donaldson was exposed four months ago as having been a spy for the British secret services for two decades, the thought that this was an execution instantly - and inevitably - sprang to mind when news of his death broke.

When Donaldson was exposed last December, the Sinn Fein leader, Gerry Adams, held a press conference in which he claimed his former colleague was not under any sort of threat from the Republican movement. Yet the grim logic of paramilitary politics always suggested otherwise. The traditional IRA punishment for informants is death. And it is telling that immediately after being exposed, and in spite of Mr Adams' assurances, Donaldson went into hiding. A journalist who tracked him down to his derelict cottage last month described him as looking like "a hunted animal".

It would be surprising if the killing turns out to have been officially sanctioned by the IRA. And it would be even more so if it could be traced back to the Sinn Fein leadership. The far higher probability is that this was a tacitly sanctioned freelance operation. Many suspect that the same informal signal of approval was given for the £26m Northern Bank robbery two years ago; an operation that is almost universally blamed on the IRA.

The implications of this killing for a resumption of devolved government in Northern Ireland could be dire. Despite the IRA's historic renunciation of violence last year and the decommissioning of its arms, the Democratic Unionist Party, now the largest party in the province, has always argued that the Republican movement remains unreformed at heart. Its leader, the Rev Ian Paisley, has always refused to join a government with Sinn Fein. Yet cracks had been developing in that rejectionism. Now Mr Paisley can use this killing to claim that a leopard never changes its spots.

The British Prime Minister had been scheduled to meet his Irish counterpart tomorrow to announce a new blueprint for reviving power-sharing in the province. The two leaders were to recommend that Northern Ireland's legislature reconvene in mid-May and elect an administration by November. Not for the first time in the history of Northern Ireland, the delicate timetable of politics has been torn up by bloodshed on the ground; and the people of Northern Ireland have been plunged, once again, into uncertain times.