The brother of one of the IRA's so-called Disappeared victims has said he is convinced the teenager's remains lie in a vast bog near the Irish border.
Oliver McVeigh said his family are sure Columba's body was dumped on the remote upland, which has been overgrown by huge forestry over the last 30 years. A renewed search has begun on the site near Bragan in Co Monaghan, near to where other digs have taken place.
Mr McVeigh said: "The family is convinced his body is there. We are 100% certain but it's just a question of locating the spot in this bog. It's still a big strain on the family and it was his mother Vera's wish to find her son that has kept us going. I can still see the anguish on her face. It will never leave me."
Columba's relatives have had their hopes raised several times over the last few years when a series of digs was ordered in the county to end their 36-year search for answers over his killing and disappearance.
The 17-year-old from Donaghmore in Co Tyrone, was abducted and murdered by the IRA in Dublin in October 1975. His mother Vera died five years ago before his body could be found.
Despite extensive searches across Monaghan, including the excavation of a grave next to Urbleshanny church, near Scotstown, last summer, his remains have yet to be found.
Fifteen men and one woman were murdered and secretly buried by republican paramilitaries during theTroubles. Nine bodies have been recovered. The painstaking search near Bragan is being carried out by the Independent Commission for the Location of Victims' Remains (ICLVR).
Hundreds of mature trees on the bog were cut down last week to open the site up for excavation and allow forensic archaeologists to inspect roots. More than 52 acres of land - mostly bleak, barren and remote bogland - has been excavated so far by the commission, set up after the Good Friday Agreement, in the search for all the Disappeared.
The remains of Columba, Joseph Lynskey, Seamus Wright, Kevin McKee, Robert Nairac, Brendan Megraw and Seamus Ruddy have yet to be located.
Frank Murray, who heads the ICLVR along with Sir Kenneth Bloomfield, signalled in February that its work could come to a halt because of a lack of new information and he appealed for support in their work. He said the renewed search of the forested bogland had been planned.