Ever since Boko Haram abducted 276 schoolgirls in Chibok in northeastern Nigeria in 2014, other groups have copied their tactics. Some 800 students have been taken from Nigerian schools since December, including 30 from a college in Kaduna last week. Who are the kidnappers? Are they criminals or terrorists – or both? And how can they be stopped? Andrew Mueller asks Chika Oduah, Nnamdi Obasi and David Kilcullen.