The leader of Boko Haram, the Islamist extremist group that abducted hundreds of schoolgirls in Nigeria six years ago, has claimed responsibility for the mass abduction of students in north-western Katsina state last week. In an audio tape released on Tuesday, Abubakar Shekau said: “Our brothers were behind the abduction in Katsina.” A large group of men armed with AK-47s overran the all-boys Government Science secondary school in the town of Kankara on Friday night, marching more than 300 students into surrounding forests.