The Real IRA has said it murdered a man who was found shot dead, stripped and bound in Londonderry on Wednesday.
In a statement to a local journalist, the organisation said it had killed Kieran Doherty, 31, a former republican prisoner from the Brandywell area.
The Real IRA claimed that Mr Doherty was a member of the dissident republican paramilitary organisation.
The body of Mr Doherty, who had a two-year-old daughter, was found on Braehead Road, near the Irish border.
Deputy First Minister Martin McGuinness said the murder was a "dirty deed" and urged co-operation with the police.
He added that the political movement linked to the Real IRA, the 32 County Sovereignty Movement, should make a statement.
In November 2009 Mr Doherty gave an interview to the Derry Journal newspaper in which he claimed he had been approached by the security service MI5 while trying to set up a cigarette manufacturing company.
According to the paper, he was repeatedly turned down for a licence by Revenue and Customs and had then been approached by an MI5 agent
"I think the whole thing is a set up in order to try and recruit informers," he told the newspaper.
In January this year, Mr Doherty again contacted the same newspaper after the police searched his home.
The search came after 500,000 euros worth of cannabis was found in a house at Carrigans in County Donegal.
Mr Doherty told the newspaper that he had no involvement with the drugs and that the house belonged to a Real IRA prisoner, Seamus McGreevy, whom he had met in Portlaoise prison.
It is understood that Mr Doherty was in prison for the armed robbery of a nightclub in County Donegal in 2001.
Mr McGreevy, who was from County Meath, took his own life earlier this year.
Police said Mr Doherty, who was due to get married in three months, had been last seen as he left home at about 2000 GMT on Wednesday.
At a press conference, the police said they were unable to confirm whether he had been abducted and whether he had been shot at the spot where his body was discovered at 2230 GMT..
Detective Chief Inspector Ian Harrison said the people who murdered Mr Doherty were criminals who had destroyed not only the victim's life, but those of his partner and his young family as well.
Speaking on behalf of Mr Doherty's family, Sinn Féin Foyle MLA Martina Anderson requested that the family be allowed to grieve in private.
"Kieran's partner Mairead and his family are traumatised and in deep shock at the sudden and brutal death of Kieran.
"They wish to be left to plan his return and burial in private and in support of each other," the MLA added.
"The family have particularly asked that media do not congregate around the family home and they thank you for respecting their wishes."
Mr Doherty's murder has been condemned by local politicians and community leaders.
The Catholic Bishop of Derry Seamus Hegarty described the killing as "brutal".
He added: "Our society has a tragic history of violence. Due to this history, we, more than many other societies, should appreciate that any attempt to drag us back to a bloody past is wrong.
"A peaceful society depends upon normal policing and justice. We have taken great strides in that direction."
Church of Ireland Bishop of Derry and Raphoe, the Reverend Ken Good, said people in the town were in shock.
"Right across the community, there is a clear sense of revulsion that another brutal murder has been carried out in this city.
"The horrific manner in which the victim's body was left at the roadside by those who perpetrated the crime is a further cause for shock and outrage," he said.