Sinn Féin president Gerry Adams immediately condemned the killing of Denis Donaldson "without reservation". Within minutes of the story breaking, he told the BBC in Belfast: "I express sympathy and condolences to the Donaldson family. I want to dissociate Sinn Féin and indeed all those republicans who support the peace process, if this man was murdered, from that killing.
"I got news of this just before the media broke the story. I contacted Denis Donaldson's family to try to make sure they got a 'heads up'. I think that the way the media and the BBC ran with this story showed no concern whatsoever for his family."
Asked who he thought might be behind the killing, Mr Adams said: "I'm not going to speculate. Denis Donaldson, as you know, worked for the British government, he was an agent for the British government. So I have an entirely open mind if he was killed or murdered, who was behind it. I would say without reservation at all that it is a killing to be condemned."
He added: "We are all living in a different era. Some of us are trying very, very hard to build a future and one in which everybody will have a share. Those who carried out this killing have no interest in that."
Asked if Mr Donaldson had been offered protection by republicans, Mr Adams replied: "How could Sinn Féin offer him any protection? One presumes that is the responsibility of the police, North and South."
IRA signatory P O'Neill issued a brief statement: "The IRA had no involvement whatsoever in the death of Denis Donaldson."
A statement from Downing Street on behalf of Tony Blair said: "The prime minister condemns this as much as anyone but equally we note Gerry Adams has strenuously condemned this and distanced peace process republicans from it. Thursday's visit [ to Armagh] will go ahead as planned."
In Dublin, the Taoiseach said: "We condemn this brutal murder. The matter is now under investigation by An Garda Síochána. We hope that whoever was responsible for this callous act will be brought to justice as soon as possible."
Northern Secretary Peter Hain said simply: "I am completely appalled by this barbaric act."
The DUP leader, the Rev Ian Paisley, said he was shocked by the killing.
He suggested republicans were to blame.
"We do not know who has done this, but a finger must be pointed towards those who were angry about what this man had done and who were enraged about what he had done."
Dr Paisley added: "I don't think this would be a loyalist attack in Donegal and I don't think it would be one of the dissenting people from the IRA, because they would not have an interest in destroying this man because they would want to hold him up as a traitor and make him an example to others."