British Prime Minister Tony Blair and Irish counterpart Bertie Ahern said they will still meet on Thursday in the Northern Ireland county of Armagh to unveil a roadmap for the province despite the furore over the death of Denis Donaldson.
The two leaders condemned the killing but emphasised the need to push ahead with plans to restore a power-sharing government here by the end of the year.
"The timing of this does suggest that whoever did this wants to derail the peace process. Our response should be to deny them what they want," Mr Blair said.
Donaldson, 55, was found on Tuesday shot dead and mutilated at his remote northwest Ireland cottage where he had fled since revealing last year that he had been a double agent in the pay of London for two decades.
Chief Superintendent Terry McGinn, who is leading the investigation, pledged: "We will leave no stone unturned to bring those people to justice."
Revealing fresh details of the crime, Ms McGinn said a window was broken and the door forced open of the dilapidated house that Donaldson had been living in near the village of Glenties, County Donegal.
A preliminary examination of his body showed he died from a number of gun shot blasts to the body, Ms McGinn told reporters at Glenties.
She said Donaldson also received other serious injuries but was unable to give further details until a full post mortem was completed later in the day.
"At this stage I am now appealing to the public to assist me in bringing the perpetrators of this crime to justice."
Members of the main Protestant Democratic Unionist Party (DUP) have accused the IRA of carrying out the murder, an accusation the Republican paramilitary group categorically denies.
Sinn Fein, Northern Ireland's largest Catholic party, also said it had nothing to do with the killing, which it condemned. One leading Sinn Fein member even pointed the finger of suspicion at the British secret service.
Donaldson had been warned by the police that his life was in danger, Mr Ahern told the Irish parliament in Dublin earlier in the day. Mr Ahern, for his part, said he had no idea who was involved.
"Whoever was responsible for this evil deed was certainly no friend of the peace process and no friend of anybody on this island," he told parliament.
The Irish leader reiterated his resolve to press ahead with plans to restore a devolved government in Northern Ireland.
But, asked whether the murder would have an impact on the process, Mr Ahern responded: "It certainly makes it more difficult."
The shock death of Donaldson aroused fresh feelings of suspicion between Protestants and Republicans, who also warned it would hurt peace efforts.