The Disappeared: Search for Columba McVeigh resumes
Another search for Columba McVeigh, who was murdered and secretly buried by the IRA, is starting in County Monaghan in the Republic of Ireland on Tuesday.
Mr McVeigh, 19, from County Tyrone, was one of 16 murder victims known as the Disappeared, who were killed and secretly buried by republicans.
Despite numerous searches, three of the victims have not been found.
Excavators are returning to Bragan Bog near Emyvale, County Monaghan, to resume the search for Mr McVeigh, the Independent Commission for the Location of Victims' Remains (ICLVR) said.
Mr McVeigh, from Donaghmore, was kidnapped in November 1975.
Several previous searches in the bog have not uncovered his remains.
At the scene
Julian Fowler, BBC News NI South West Reporter
The latest search at Bragan Bog began last September and was temporarily stopped in November, as the weather closed in.
Investigators from the Independent Commission for the Location of Victims' Remains have identified an area of about an acre where they believe Columba McVeigh was buried.
They are about halfway through the search.
The weather conditions and the difficulties of the terrain means investigators have not put a timescale on their efforts - they will stay as long as it takes to complete the search.
Jon Hill, senior investigator with the ICLVR, said it had taken some time for the bog to dry out sufficiently to enable the use of machinery for Tuesday's search.
"We've managed to do some preparatory work in advance of today and so we're able to get down to the search proper straight away," he added.
"As I explained back in September we are searching an area of about one acre in total, which is smaller than some we have worked on in previous Disappeared searches but this is challenging terrain."
Mr Hill said he would not put "a precise time frame" on how long it would be at the site.
"Obviously our hope is that we find Columba at an early stage but as ever we'll have to wait to see," he said.
"What I can absolutely guarantee is that the highly skilled contractors and forensic archaeologists working here will do everything they humanly can to get the result that we all know will mean so much to the McVeigh family and all those who support them."
Last month, it emerged a reward of almost £50,000 is being offered for new information that results in finding the bodies of the Disappeared.
The anonymous donation of $60,000 (£47,191) has been given to the independent UK charity Crimestoppers.
Crimestoppers takes calls anonymously by telephone or accepts information through an anonymous online form.
It says any information that it receives will be passed only to the Independent Commission for the Location of the Victims' Remains (ICLVR).
The ICLVR was set up to obtain information that may lead to where the bodies of the Disappeared are buried.
Information it receives is strictly confidential and is not passed to other agencies or used in prosecutions.