The attackers in San Bernardino, Orlando and New York all had expressed support for the Islamic State, and they and the Boston bombers were devotees of the voluminous online work of Mr. Awlaki, who was killed in an American drone strike in 2011. He has led many people down the jihadist path, and not just in English-speaking countries. The Counter Extremism Project, an advocacy group based in Washington, said Wednesday that it had counted 88 “extremists” who had been influenced by Mr. Awlaki: 54 in the United States and 34 in Europe.