It’s another one of those baffling mysteries. Muslims attack non-Muslims while shouting Allah Akbar and no one can figure out a motive. Agnes Thibault-Lecuivre, a spokeswoman for the Paris prosecutor’s office, said the two victims had been wearing Jewish skullcaps, and the attacker was detained after a chase through the synagogue. The Simon Wiesenthal Center, which monitors anti-Semitic incidents worldwide, said in a statement that the assailant screamed “Allah-u-Akbar” — or “God is great” — during the attack. The rabbi, who is in his late 40s, was recovering from surgery for a neck injury and his 18-year-old son sustained lesser...