Three insurgents, including the bomber, attacked Potiskum’s College of Administration and Business Studies around 8 a.m., Marcus Danladi, the police commissioner for Yobe state, said by phone from Damaturu.

"Our men arrested one of the fleeing insurgents and recovered one AK47 rifle and two rounds of ammunitions," he said.

While no group immediately claimed responsibility, Islamist militant group Boko Haram has been waging a six-year campaign in northern Nigeria to impose its version of Shariah law on Africa’s largest oil producer. At least 5,500 civilians were killed by Boko Haram between last year and early 2015, according to rights group Amnesty International.

"I heard a loud explosion and saw black fumes from the school rising to the sky," Faruk Ismail, a resident, said by phone from Potiskum. "While we are running away for our dear lives I heard gun shots from the direction of the school."