Denis Donaldson was in the headlines when Stormont was suspended in 2002 - and his death could jeopardise attempts to revive the political institutions.
Denis Donaldson was found shot dead in County Donegal
The murder of the double agent - who was at the centre of the spying allegations that collapsed power-sharing - comes just 36 hours before the prime minister and the taoiseach were due to unveil their joint blueprint to restore Stormont.
That blueprint was to be unveiled, amid much controversy, in Armagh.
No doubt the prime minister, who has been toiling over his words, will be reworking them to reflect the latest developments.
But he will insist on pursuing the plan which has angered Sinn Fein.
Republicans are unhappy that the assembly plan more closely reflects the DUP's demands for a trust-building assembly that operates without an executive.
While the DUP will want to push for more time, republicans have condemned plans to have the assembly operate for six months without ministers.
The plan is said to involve having Stormont opened again next month with a view to electing a first and deputy first minister to head a new executive.
While there was no expectation such a scenario would work within the six weeks allotted, this was meant to be the first of two attempts this year over the six-month life of the assembly.
The DUP would have come under serious pressure to support the spring executive but now may add Denis Donaldson's murder to their list of concerns.
While it is too early to conclude who murdered Denis Donaldson, there can be no doubt that his death is casting a dark shadow over the peace process.
Despite the fact that so many informers over the years met brutal ends, his murder has been almost as shocking as the news that he was a long-term informer.
Donaldson was outed in a phase in the peace process when the IRA had announced it was pursuing political ends only.
A blueprint to restore Stormont was due to be unveiled
He had clearly hoped that he might be allowed to live in peace, as he had pleaded just days ago when a reporter tracked him down.
Even so, there was surprise that someone with his past was living in Donegal in a run-down cottage in a republican heartland.
Sinn Fein has said it knows nothing about the death and Gerry Adams has disassociated republicans who support the peace process from the killing. The IRA has denied involvement.
If the Provisional IRA was not involved, however, it is possible that an individual member - or indeed a dissident organisation - was involved.
The DUP leader Ian Paisley suggested he gave no credence to denials from republicans, including Gerry Adams.
While awaiting the pronouncements of the police, unionists will be ever more anxious about the prospect of power-sharing with republicans.
What is certain is that Mr Blair will find it difficult to wave his finger at unionists and demand power-sharing while the circumstances surrounding Mr Donaldson's death are unclear.
Tony Blair had no doubt hoped to remind everyone of the promise of the Good Friday Agreement and Easter 1998 but has found that, seven years on, the brutalities of the past have not been laid to rest and continue to blight his carefully-laid plans for Northern Ireland.