Published Monday, 01 March 2010
Kieran Doherty was found dead, stripped with his hands tied behind his back on the Braehead Road just outside Derry on Wednesday night.
The Real IRA said they "executed" Mr Doherty and claimed he was a member of the paramilitary organisation.
The victim's family have rejected allegations that the 31-year-old was an informer or a drug dealer.
They admitted Mr Doherty had served time in prison, reportedly for a robbery in Co Donegal, but claimed he had been trying to get on with his life.
They alleged he had been under "continuous harassment" from the secret service MI5 in the months before his death.
"They tried to make out he was an informer", Mr Doherty's sister, Leeane Doherty, told UTV.
"Kieran would not speak out, so they had to take away his life. I believe MI5 was involved and somehow maybe linked with the Real IRA because Kieran would not speak."
The family have called for an inquiry into the security service's involvement with Mr Doherty.
"My family would like to see Peter Robinson and Martin McGuinness investigate what role MI5 had in the brutal murder of my brother", Ms Doherty said.
The family have appealed for no retaliation during the funeral service held on Monday morning.
Hundreds of people attended Requiem Mass at Long Tower Church, including senior republican political figures and people linked to dissident republican groups in the Derry area.
Local priest Father Roland Colhoun called on the Real IRA to reflect on their actions.
"An unexpected death inflicted by a gun at the side of a lonely road. No-one would wish for that, and no-one should inflict that on another person.
"Our society has a lot of soul-searching to do in a week like this. But we ask you, the people who brought death and heartbreak to this community, will you rejoin the civilised way of life that God calls you to?" he asked.
"Kieran's family asks that there be no retaliation so that no other family will have to experience this trauma," he added.
Mr Doherty's coffin was draped in an Irish tricolour flag as it was taken for burial to the City Cemetery in Derry.
The victim, from the Brandywell area of Londonderry, was the father of a two-year-old daughter and was due to marry his partner, Mairead, in three months.
His murder provoked outrage among politicians and community leaders.
The Derry trade union movement is organising a rally in Guildhall Square on Tuesday afternoon to protest against the murder and call on all armed gangs to stop violence.
During ministerial question time, the deputy First Minister said the dissidents were no army, just a gang of people with no lives who represented nobody.
"This is a gang of people who are opposed to peace, who are opposed to justice and who are opposed to political progress and these are a gang of people who believe that it's a legitimate political objective to destroy all of the good work that many of us in this house have been engaged in with the assistance of others over the course of the last 15 years," Mr McGuinness said.
"We're moving forward, we're not going to be diverted and each act they carry out, instead of us becoming weaker, we will get stronger, stronger than ever before."