The man, named locally as Robert Magill, was ambushed by two men who lay in wait for him in woodland in a suburb of Rickmansworth, Hertfordshire. They shot him several times in the chest with a shotgun.
The two men were seen escaping along a footpath to junction 18 of the M25.
As police investigated the victim's background, neighbours spoke of the strange lifestyle of the man who had lived for six months in the former concrete munitions store with his wife and children. They nicknamed the bungalow Fort Knox because Mr Magill had gone to such lengths to seal it off. The house, garage and trees were fitted with video cameras. A high fence was built around it, electronic devices controlled the gates and a Union flag flew from the rooftop.
The alarm was raised by a neighbour who heard shots and rushed from her home to see the victim lying in a pool of blood. Inspector Ian Noble, spokesman for Hertfordshire police, said: 'We are interested in talking to anyone who has seen this man walking his dog before. This was clearly a well-planned execution.'
Mr Magill's wife, Elaine, and their children, Dustin and Tanya, were under police protection last night.