Then came the next nightmare: that the assassination, an act carried out both in cold blood and in great hatred, might sabotage all the hopes for peace and political progress.
This was plainly its aim, at a time when most key Northern Ireland politicians were in Washington gearing up for a negotiating session later this month aimed at breaking the deadlock on decommissioning.
Those who planted the bomb that took Rosemary Nelson's life clearly wished both to affect the future, by derailing the peace process, and to punish her. In recent years she had acted not just as an ordinary defence solicitor, but as a champion of the beleaguered Catholics of Portadown, Co Armagh.
The attack came at lunchtime yesterday in Lurgan, not far from Portadown. The booby trap, which is assumed to be the work of loyalists, detonated as the solicitor drove her BMW car away from her home at Ashford Grange. The bomb went off as she approached a junction, possibly activated as she applied the brakes. One of the firemen who was at the scene said they found Mrs Nelson with "horrendous" injuries. She was cut free and taken to the intensive-care unit of a nearby hospital, where she died at 3.10pm. Her eight-year-old daughter, Sarah, was at school not far from the scene of the explosion, while her two sons were abroad skiing.
Mrs Nelson was unusual among Northern Ireland solicitors in that she went beyond the normal run of court work carried out for individual clients. She went on television to criticise both loyalist organisations and the security forces.
She was particularly critical of the Royal Ulster Constabulary, which, she alleged, colluded with extreme loyalists and paid less than due regard to the rights of nationalists.
Within hours of her death this theme had been taken up by local nationalists, who made it clear that they blamed not just loyalists but also the security forces for her killing.
Graffiti appeared in Lurgan proclaiming: "Rosemary Nelson - the people's voice - murdered by the RUC/RIR", a reference to the locally recruited section of the army, the Royal Irish Regiment. Around 200 people carrying black flags and placards calling for the RUC's disbandment marched to Lurgan RUC station, where they staged a largely silent protest.
They passed close to the spot where in July 1997 the IRA killed two RUC constables, John Graham and David Johnston. When a leading local republican, Colin Duffy, was charged with their murders, Mrs Nelson mounted a spirited defence, in the courtroom and in the media, and the charges were dropped.
A year ago a United Nations report documented Mrs Nelson's allegations that she had been the target of death threats. She also accused an RUC officer of spitting in her face and calling her a "Fenian" sympathiser, and another of striking her on the head with a riot shield during a street confrontation.
The Graham and Johnston killings caused Tony Blair publicly to question the worth of the peace process. They also raised the temperature in the run-up to the annual July Drumcree marching dispute. Rosemary Nelson's death seems bound to have a similar effect. With Drumcree only months away, no agreed formula has emerged to avoid a repeat of the highly damaging episodes which annually sour community relations. Although some have been hopeful that progress in averting another confrontation could be made, her death will worsen the atmosphere and add to the huge repository of mistrust.
Last night, nationalist anger spilled on to the streets of Lurgan as crowds of youths attacked the RUC and Army near Mrs Nelson's bombed-out car, and petrol bombs and stones were thrown on the nationalist Kilwilkee estate.
There were signs that within hours of her death Mrs Nelson was being elevated to the status of martyr.
Dolores Kelly, the nationalist deputy mayor of Craigavon, who went to school with Rosemary Nelson, said: "She was very much respected and was seen as a heroine in the local community... She was often the voice of justice, many felt sometimes the voice in the wilderness."
From Washington, the First Minister designate, Ulster Unionist leader David Trimble, said he was horrified by the killing. He called on loyalists "to realise that there is no point whatsoever in violence".
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