What makes the gruesome killing of Father Jacques Hamel all the more frightening is the impossibility of preventing such crimes. An elderly priest saying morning Mass for a small weekday congregation could hardly be more vulnerable. His quiet Normandy town of Saint-Etienne-du-Rouvray would never stand out as high risk. After previous outrages by terrorists claiming allegiance to Islamic State (IS) such as the attack in Nice earlier this month, French security measures were subject to minute scrutiny by politicians and the media, and by the security agencies themselves, to pinpoint mistakes and learn lessons.