Gerard Evans from south Armagh was last seen alive in March 1979 while hitching a lift from a dance in Castleblayney.
It is believed that Mr Evans was killed and his body buried at Carrickrobin in Co Louth.
A person claiming to be part of the 12-man gang who shot Mr Evans contacted the Sunday Tribune newspaper with information that has led to this latest search.
The Independent Commission for the Location of Victims Remains said it was returning to the site to try to locate the remains of Mr Evans.
Earlier this year, a geological survey was carried out on the farmland, 3km from Hackballscross, and drainage systems put in.
Diggers have also moved on to the site.
A spokesman for the Commission said the excavation could last for a considerable time. Recent heavy rain fall could also hinder the search.
Findings from an initial search have been examined and the follow-up search has begun.
Meanwhile, new legislation comes into force in Northern Ireland today, which is hoped can benefit the families of the 'disappeared'.
Relatives of those thought to have been murdered and secretly buried will now be able to have the presumed death of their loved one legally acknowledged by the courts.
Northern Ireland Finance Minister Sammy Wilson said the act is long overdue.
