An attack on a major synagogue in Michigan has resurrected fears about violence at houses of worship. Attending a religious service remains a remarkably safe thing to do. But Thursday's attack has made clergy around the world anxious about targeted ambushes. There have been different kinds of violence over the years across the world. A wide range of religious buildings have been targeted. Sometimes there is an explicitly political motive. Other times it is not clear why a perpetrator carried out such a devastating tragedy.