The Independent Commission for the Location of Victims' Remains (ICLVR) has renewed an appeal to locate the remains of Army officer Robert Nairac.
Captain Nairac was abducted by the IRA while on an undercover operation in a pub in south Armagh in 1977.
He was then taken to Flurry Bridge in County Louth where he was beaten and shot dead.
Lead investigator Geoff Knupfer says his focus is to end Capt Nairac's two sisters' 45 years of suffering.
"As I have said before that while we are looking for the physical remains of Robert Nairac, he is not the focus of our work. That is firmly fixed on ending the near half century of suffering of his two sisters Rosemonde and Gabrielle", he said.
"Robert Nairac's case is the only one of the original Disappeared ICLVR cases in which we have not carried out a physical search because we have not had enough credible information to go on."
Robert Nairac was one of 16 people abducted and murdered by republicans during the Troubles.
His body has never been found and there has been much speculation about the disposal of his remains.
The ICLVR was set up to find the bodies of the Disappeared.
Mr Knupfer said he is confident that there are people who know where Robert Nairac's remains are buried.
He emphasised that any information given to to the commission is treated in the strictest confidence and that its sole objective is to allow families to bury their loved one, just like any other family that has suffered loss.
"There are four outstanding cases: Joe Lynskey, Columba McVeigh, Robert Nairac and Seamus Maguire. Four families still waiting," added Mr Knupfer.