Dublin, Ireland ? A former Sinn Fein official recently exposed as a British spy was found fatally shot Tuesday after apparently being tortured, police said – a slaying certain to send shock waves through Northern Ireland’s peace process.

Denis Donaldson was Sinn Fein’s former legislative chief in the failed power-sharing government of Northern Ireland. He admitted in December he had been on the payroll of the British secret service and the province’s anti-terrorist police for two decades. He went into hiding because the traditional Irish Republican Army punishment for informing is death.

But the IRA denied responsibility in a one-line statement. “The IRA had no involvement whatsoever in the death of Denis Donaldson,” the outlawed group said.

Irish Justice Minister Michael McDowell said the 55-year-old Donaldson had been tortured before being killed inside his isolated home near Glenties, County Donegal, in northwest Ireland. He was last seen alive Monday walking in the village.

“His right forearm is almost severed,” McDowell said. “He was shot in the head and mutilation was done to his body. It’s a murder we’re dealing with.”

Irish Prime Minister Bertie Ahern and British Prime Minister Tony Blair both condemned the murder.

The killing comes at a pivotal moment in Northern Ireland’s 13-year-old peace process.

On Thursday, Blair and Ahern are to travel to Northern Ireland to reveal a new blueprint for reviving a Protestant-Catholic administration, the intended cornerstone of the province’s 1998 peace accord. The plan – 3 1/2 years of diplomacy in the making – would call for Northern Ireland’s legislature to reconvene in mid-May and face a Nov. 24 deadline to elect an administration.

Sinn Fein leader Gerry Adams – who in December initially defended Donaldson as an innocent man, then outed him as a British spy – said he did not know who killed him. He suggested it might have been the work of IRA dissidents opposed to Sinn Fein’s peacemaking efforts.

But Ian Paisley, whose Democratic Unionist Party represents most of Northern Ireland’s British Protestant majority, said someone within IRA ranks was the most likely culprit. “There is a finger-pointing tonight at IRA-Sinn Fein,” he said.