In a sense the Tunisian gunman Seifeddine Rezgui was Everyman. He was not known to be a particularly devout Muslim; an internet video shows this healthy young engineering student demonstrating his break-dancing; he wore a Real Madrid shirt. Yet he has murdered 38 innocent holidaymakers of whom at least 30 are thought to be British. As a result he has nearly shut down the Tunisian tourist industry. This is political and economic terrorism at its most cruel and destructive, and the hardest to combat. How many people there are like Seifeddine Rezgui in Britain or elsewhere in Europe, nobody knows, and picking them out is a nightmare for the police and security services. But they have to try.In addition to the obvious difficulties, they also face a level of scepticism and even hostility from