And there have been suggestions that it attacks certain mosques because members have spoken out against it and helped federal officials with their crackdown. Analysts say attacks are aimed at instilling fear in the local population to prevent cooperation with the government.

Mohammed Yusuf, a charismatic young cleric, founded Boko Haram 12 years ago as part of his push for a pure Islamic state in Nigeria. He was killed in 2009, but his group lived on.

Boko Haram became more violent after his death as his supporters vowed to strike back. Human Rights Watch estimates that in the past five years, more than 3,000 people have been killed in the violence.