The militants stormed the home of Kamel Youssef, a plumber, on Thursday and shot him to death in front of his wife and children in the town of el-Arish, said two security officials and the priest.The officers spoke on condition of anonymity because they were not authorized to talk to reporters, as did the priest, fearing retaliation by militants. The priest said initial reports that the man’s daughter was kidnapped and killed turned out to be incorrect. No militant group has claimed responsibility for the attack. But Egypt’s Islamic State group affiliate, which is based in the Sinai Peninsula and which in December carried out a devastating suicide bombing against a Cairo church, vowed in a video earlier this week to step up attacks against the embattled Coptic Christian minority. It described them as “infidels” empowering the West against Muslims.