Ivan Telzer was in the middle of his prayers at the Heaton Park Congregation’s synagogue in Manchester, England, when people started shouting. “Shut the doors!” they cried. “Shut the doors!” Mr. Telzer said he could hear someone pounding from outside, trying to get in the synagogue. Some of the roughly 20 congregants who had gathered for morning services on Yom Kippur, including the rabbi, used their bodies to barricade the large black doors, he said. The police later identified the person outside as Jihad al-Shamie, a Syrian-born British citizen who carried out a terrorist attack on Thursday in a city...