The gang focused on a car driven by a credit union official, who had collected €4,000 from the Lordship premises.
But a second car had collected a further €25,000 to €30,000 from the three other credit unions in the area.
They planned to travel together in the two cars, accompanied by a garda escort, to lodge the money in a night safe at a bank in Dundalk town.
The murderers used their car, a dark coloured Volkswagen Passat to block the exit from the car park.
As the convoy was leaving, the two detectives spotted the blocked exit and Garda Donohoe stepped out of the patrol car to investigate.
But he was shot at almost point blank range in the head with a long-barrelled shotgun when he approached.
The Passat was found burnt out yesterday afternoon on the Keady to Newtownhamilton Road in south Armagh, near the Darkley Pentecostal church where three people were shot dead and seven injured by the INLA as they prayed there in 1983.