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Nava Silton, youth director at the Jewish Center, was packing up and readying to leave the Modern Orthodox synagogue on the Upper West Side of Manhattan after running a program for the Jewish fast of Tisha B’Av on Thursday. She chatted with a friend, a 50-year-old man, who was leaving after praying the afternoon service of Mincha. Soon thereafter, she heard that someone had been stabbed outside the synagogue. After running down the stairs and asking security what happened, she learned that it was the man with whom she had been talking and that he had just been taken to the hospital,...