He described the pain felt by the Co Tyrone teenager's family as "heartrending", and also issued a fresh plea for information to find the final three Disappeared victims that have not yet been found, including former monk Joe Lynskey and soldier Robert Nairac.
All three were among 16 murdered and secretly buried by the IRA and INLA during the Troubles.
"I once more appeal to the conscience of anyone who has information that might help find Columba, Joe Lynskey or Robert Nairac, to come forward to the Independent Commission for the Location of Victims' Remains, so that the remaining families can experience the consolation of being able to offer their loved ones a Christian burial," he said.
"The slightest clues could lead to them. Please help end their heartache.
"I appeal also to those carrying long-held secrets, from what were awful, terrible times, to share what information they have - they may hold the key to unlocking the uncertainty and grief of those families who are still desperately waiting and hoping.
"I ask everyone to keep the families of the Disappeared in their prayers during these difficult days."
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