Police are investigating whether loyalist paramilitaries abducted and murdered a former associate over money he owed to them.
Mark Gourley, once described in court as a "loyalist foot-soldier", went missing in March 2009.
The 36-year-old was sentenced to 18 months probation in 2005 for his part in an attack on an undercover police officer who was investigating UDA extortion in Carrickfergus, Co Antrim.
Detectives are now examining whether his former associates were responsible for murdering him more than four years ago. Mr Gourley's body has never been recovered.
A senior detective said he was convinced Mr Gourley had been murdered and declined to rule out paramilitary involvement in his death. A murder investigation has been launched after police received fresh information regarding Mr Gourley's disappearance. However, police have declined to elaborate on the nature of that information.
"At this stage I can't go into the detail of that information but it convinces me and my team that Mark Gourley has been murdered and we are appealing for anyone to give us assistance on that," said Detective Inspector Neil McGuinness.
It has been claimed Mr Gourley had run up debts to loyalist paramilitaries in south Antrim.
Mr McGuinness would not be drawn on those allegations.
"All lines of inquiry are being looked at and will be followed up appropriately," he said.
The senior detective added: "I can certainly say that at the time of his disappearance Mark Gourley was not an active member of a paramilitary organisation."
Mark Gourley, from the Castlemara area of Carrickfergus, was last seen in the Burney's Lane area of Glengormley at about 2.30pm on March 7, 2009.