Police say DNA tests have confirmed that the body found in a car taken from the Newry canal on Saturday was that of the missing Armagh man Gareth O'Connor.
They say the theory that he was murdered is still a major line of enquiry.
Acting on information, police searched part of the Newry canal, where they found a blue car with a decomposed body inside.
A post mortem examination proved inconclusive, but with the assistance of DNA tests, police have now confirmed the body is that of Mr O'Connor.
Mr O'Connor disappeared in May 2003 and police believe he was abducted and murdered by the IRA.
At the time of his disappearance, Mr O'Connor, a 24-year-old father of two from Armagh City, had been charged with membership of the Real IRA.
He vanished after failing to report to Dundalk Garda Station as part of his bail conditions.
His family and police suspected that the father of two was abducted and killed by the IRA, which has denied any involvement.
Dominic Bradley of the SDLP said there could now be no excuse or justification for not delivering up the bodies of the other disappeared. He said if the Provisional IRA was really going out of business, it needed to sort out its unfinished business now.
