Mr. Yusuf was forced to flee to Saudi Arabia, partly because of the attacks. But he had powerful connections in the state government at the time and was allowed to return. His disciples, including Mr. Shekau, benefited from the same benevolence. The group that became Boko Haram coalesced in Maiduguri in the mid-2000s; Mr. Yusuf attacked schools and the government, and Mr. Shekau, his lieutenant, amplified the message in the neighborhoods.