Ex-British agent Denis Donaldson died from a shotgun wound to the chest, post mortem results have indicated.
Other injuries to his body were consistent with shotgun blasts, including a severe injury to his right hand, said Irish police.
The IRA has denied involvement in the murder of the ex-Sinn Fein man, who was found shot dead following a break-in at a house in County Donegal on Tuesday.
The Irish premier said Mr Donaldson was warned his life could be in danger.
Bertie Ahern said police became aware of Denis Donaldson's whereabouts in January, but he did not request any protection.
Irish police, who have appealed for information, said he died from a number of shotgun blasts to the body.
Mr Donaldson, 56, was found dead in a room in a remote cottage near the village of Glenties in County Donegal, at about 1700 BST.
He had been expelled from Sinn Fein in 2005 after admitting he was a paid British spy.
Mr Donaldson was found dead in a remote cottage in Donegal
At a news conference in Donegal on Wednesday, Chief Superintendent Terry McGinn refused to be drawn on details of the killing or if there had been a specific threat to Mr Donaldson.
She said the door had been forced and a window broken in the property. She added they were keeping an open mind about the inquiry.
"An Garda Siochana will leave no stone unturned in bringing these people to justice," she said.
"But I need the help of the public. I need the help of any person who may have been in this area in the last few days and who may have spoken to Denis Donaldson or who may have spent time with him. "
Earlier, Mr Ahern outlined the actions of Irish police when they became aware of Mr Donaldson's whereabouts in January.
"Garda visited him in the light of the public attention which he'd received and they advised him that because of his circumstances there was a perceived element of threat to his life," he told the Irish Parliament.
"They offered advice on personal security and the telephone number of Glenties Garda station in case he had any concerns.
"On an on-going basis, therefore, the house where he lived received passing attention from the Garda. I should note that Mr Donaldson did not at any time request Garda assistance or protection since that period in January."
He said the victim's right hand was "practically severed at the wrist".
Mr Donaldson's death came two days before a planned visit to Northern Ireland by Mr Blair and Mr Ahern to unveil their blueprint for reviving the assembly at Stormont.
However, the British and Irish prime ministers have insisted they will not let the murder derail the political process.
Mr Blair said the killing made it "even more important that we stand firm on the peace process".
"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," he said.
Mr Blair described the killing as "a serious, appalling, barbaric act".
TIMETABLE OF EVENTS
4 October 2002: Three men including Mr Donaldson arrested following raid on Sinn Fein's Stormont office. Power-sharing executive collapses and government restores direct rule to NI a week later
8 December 2005: Charges against three men dropped "in the public interest"
16 December 2005: Sinn Fein says Mr Donaldson was a "British agent" and expels him from the party: he later says he worked as a spy since the 1980s
Government and police reject the party's claim raid was politically motivated
4 April 2006: Donaldson found shot dead in County Donegal
Northern Ireland Secretary Peter Hain said the murder would not deflect the government from the political process.
"If we were to allow the peace process to be derailed, we'd be giving into violence, in this case a grisly, gruesome murder, perhaps deliberately calculated to throw a spanner in the works and make the political process difficult," he said.
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.
Hugh Jordan, chief reporter of the Sunday World newspaper, interviewed Mr Donaldson last month, after tracking him down.
He said that when he spoke to him he had "the sort of look of a hunted animal, but after a while he settled down".
"I did ask him about his future and ask him what the future held for him now and he said 'this is it' and I said 'Donegal?' and he said 'yes'.
"I asked him would he ever go back to Belfast and he said 'I don't know'."
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 he admitted being recruited in the 1980s as a paid British agent.
He said there had not been a republican spy ring at Stormont.