Republicans helped to refine a search area ahead of a fresh attempt to find a teenager who was disappeared by the Provisional IRA more than 40 years ago.
Columba McVeigh, 19, from Donaghmore, Co Tyrone, was abducted, murdered and secretly buried on November 1, 1975.
Excavators started digging in bogland across the Irish border in Co Monaghan on Monday morning.
Four previous searches in the Bragan bog near Emyvale have not uncovered his remains.
He is one of three of the ‘Disappeared’ victims of the Troubles whose bodies have still to be found.
Jon Hill, senior investigator with the Independent Commission for the Location of Victims Remains (ICLVR), said they were helped to identify an area to search.
He said previous search attempts have helped restore the area to how it would have looked in 1975.
“Since 2013 when we concluded the last search, we have re-evaluated all the information we had and we have gone back to the drawing board and encouraged through the republican movement any people that might have information about this location to revisit, and to revisit in their minds as well,” he said.
“The site has been restored pretty much to how it was back in 1975, that’s the coincidence of us taking down the trees down during a search we did further down. So, we encouraged them to come back and, as a result, an area has been designated as worthy for searching for him.
“That area has been marked out and that is where we are now undertaking our search.”
It is expected to take around two weeks for the excavators to remove the top level of peat from the section they plan to search. After that, ICLVR workers, including archaeologists, will examine an area measuring approximately 40 metres x20 metres.
“We are hopeful, we wouldn’t be here if we weren’t hopeful, but you can’t raise expectations too high too early,” Mr Hill added.
“The information is good that has brought us here, the equipment that we are using is right, the knowledge and experience is good and the staff are good, so we have everything in our favour.
“What we need next is to be in the exactly the right place.”
The latest search for Mr McVeigh started on the 21st anniversary of his father’s death.
Paddy McVeigh died without ever knowing where his son’s remains are buried. His wife Vera passed a decade later, again without answers.
The teenager’s brother Oliver McVeigh said the new search was hard for the family, given the number of unsuccessful digs. He said, though, that they were “cautiously optimistic”.
He told Press Association that he intends to attend the search, but not every day as he does not want his presence there to be a distraction.