Kieran Doherty, 31, was stripped and tied up and left in a remote lane on the outskirts of Londonderry close to the border with the Republic of Ireland on Wednesday night.

His family alleged he had been under "continuous harassment from the secret service MI5 in the months before his death. "We repeat that he was neither an informer nor a criminal. He was never in his life associated with drugs," the relatives' statement said.

The victim was the father of a two-year-old daughter and due to marry partner Mairead in three months. He was from the Brandywell area of Londonderry.

Mr Doherty was discovered by a passer-by on the Braehead Road on the outskirts of the city on Wednesday night.

The family admitted he had served time in prison, reportedly for a robbery in Co Donegal in the Irish Republic, but claimed he had been trying to get on with his life.

"Kieran was under continuous harassment by MI5 in the months before his death," they alleged. Repeated attempts were made to recruit him as an informer. He rejected all these attempts." They asked for an inquiry into whether MI5 was involved.

The statement added: "Kieran was a good and decent man trying to lead a normal life, looking forward to getting married."

They said in the last months of his life he was stressed, lost weight, was treated in hospital for depression and was sometimes almost unable to speak to his partner.

Police have confirmed post-mortem reports suggested he died from gunshot wounds. His body has been returned to his family.