Former senior Sinn Fein member Denis Donaldson has been found shot dead in the Irish Republic.
Mr Donaldson was expelled from the party last December after admitting he was a paid British spy for 20 years.
The IRA issued a statement saying it had "no involvement whatsoever" in Mr Donaldson's death in County Donegal.
Irish prime minister Bertie Ahern described the death as a "brutal murder", while NI Secretary Peter Hain said it was "barbaric".
Sinn Fein leader Gerry Adams said he wanted to "disassociate (his party) and all republicans who support the peace process from this killing".
The death of Mr Donaldson came hours before a planned visit to Northern Ireland by Prime Minister Tony Blair and Mr Ahern to unveil their blueprint for reviving the assembly at Stormont.
A Downing Street spokesman said that Mr Blair "strongly condemned" the killing and had noted Mr Adams' statement of condemnation.
TIMETABLE OF EVENTS
It is understood that Irish police found two shotgun cartridges close to the body of Mr Donaldson at his remote cottage in County Donegal.
Irish justice minister Michael McDowell said he understood that Mr Donaldson had extensive damage to his right arm.
Mr Donaldson moved out of his Belfast home last December, and had been living in the run-down cottage which had neither electricity nor running water.
His body was found at the cottage near the village of Glenties at about 1700 BST on Tuesday.
A large area surrounding the property has been cordoned off by Irish police for scientific examination.
One of Mr Donaldson's nearest neighbours, who lives more than a mile away, said he had last seen him driving past on Tuesday morning.
Mr Donaldson had been Sinn Fein's head of administration at Stormont before his 2002 arrest over alleged spying led to its collapse.
Mr Donaldson and two others were acquitted of charges last December "in the public interest".
One week later, Sinn Fein expelled him from the party.
At the time, he told a news conference that he was recruited in the 1980s as a paid British agent and deeply regretted his activities.
He said there had not been a republican spy ring at Stormont.
Gerry Adams told the same news conference that Mr Donaldson was not under any threat from the republican movement.
Mr Adams said on Tuesday he had spoken to the Donaldson family just before news of his death broke.
He said he was not prepared to speculate on who might have been responsible.
"It has to be condemned. We are living in a different era, and in the future in which everyone could share," he said.
"This killing seems to have been carried out by those who have not accepted that."
DUP leader Ian Paisley said Mr Donaldson's death would be a setback for the political process.
"If this man has been murdered because of his connection with the IRA/Sinn Fein, and because of the past happenings, then it strikes a blow at what the two governments are trying to do," he said.
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