A dig for the remains of a teenager murdered by the IRA is proving one of the most challenging undertaken by experts searching for the Disappeared victims of the Troubles, a senior investigator has said.
Jon Hill, from the Independent Commission for the Location of Victims' Remains (ICLVR), outlined the issues his team has encountered as the search for Columba McVeigh entered a new phase.
"The searches are all challenging, they are all difficult, but this one is particularly difficult," he said.
Mr Hill's assessment came as new drone footage of the dig site in bogland in Co Monaghan captured the scale of the operation to find Mr McVeigh.
The 19-year-old from Donaghmore, Co Tyrone, was abducted, shot and secretly buried in November 1975.
Republicans with knowledge of the crime have told the commission, in confidence, that he was buried across the border in Bragan bog near Emyvale.
Four previous searches in the bog have not uncovered his remains.
The fifth search commenced last month. For the last two weeks excavators have been removing the top level of peat from the section they plan to search.
Now an ICLVR team of forensic archaeologists and technicians will comb an area measuring about 60 metres by 20 metres - a task that could take months.
Mr Hill said peat excavated during previous searches ended up being dumped on the area now being examined.
"The dilemma we faced here is we had up to two metres of peat that had been redeposited on the site from various searches around various areas," he said.
"Because of the amount of disturbance this ground has undertaken over the years it makes it extremely challenging."
Mr Hill said techniques used on previous Disappeared searches, including cadaver dogs and geophysical surveys, were not beneficial for the current dig as there was too much previously disturbed earth.
The senior investigator said the volume of wet peat also presented a health and safety issue, as the excavators could not track across the land without sinking into it. The team is using special mats to move the machines over the site.
Part of the area was examined in the first search for Mr McVeigh in 1999 - an operation undertaken by the Garda.
Mr Hill said it was worth re-examining the ground because techniques now used by the commission were not available to gardai 20 years ago.
Mr McVeigh's brother Oliver said his overriding emotion as the dig progressed was one of "frustration".
"It's really a waiting game," he said.
"You have to be learn to be patient. We have to be getting closer, but again we could be left disappointed.
"I'm searching for him from when I was 37, I'm now 57. That's only the searching - not the length of time he's been missing. The physical search to find his body is 20 years old.
"My mother is dead 11 years and my father didn't even know about the searches. I was young when they first started searching and I was helping my mother along and now I'm 57.
"When you put things into perspective like that it's crazy.
"The one positive thing is there's someone there digging and that's the only chance they have of finding him. Because if there's nobody there digging, they won't find him."
Three of the 16 people classified as the Disappeared remain missing.
As well as Mr McVeigh, they are former monk Joe Lynskey and soldier Robert Nairac.
The ICLVR can be contacted confidentially by telephone on 00800 55585500; International +353 1 602 8655.