Attackers with guns and explosives raided a jail in a restive Nigerian city on Sunday, leaving four prison guards dead and freeing 40 inmates, authorities said. The raid occurred in the northeastern city of Damaturu, previously hit by heavy violence blamed on Islamist group Boko Haram and which last week saw gun battles between security forces and extremists that killed at least 40 people. It was not yet known who the attackers were. Police claimed the jail was not holding any Boko Haram members. \"Forty escaped,\" said state police commissioner Patrick Egbuniwe, adding that one of the inmates was re-arrested and another was killed, though he did not know if police or the assailants shot him. The raid saw attackers throw two explosives and open fire, forcing their way into the central prison in Damaturu, a prison officer said. \"A prison guard responded and there was a shootout, but the attackers overpowered the prison guards and broke into the prison,\" said the officer, who was not there at the time but gathered information from colleagues. \"Four guards were killed and an unspecified number of the attackers were shot, but their comrades took them away.\" Boko Haram\'s insurgency has killed hundreds, mainly in Nigeria\'s north. The group has previously targeted prisons and freed Boko Haram members. On Friday, President Goodluck Jonathan fired his national security adviser and defense minister amid growing fears over spiralling violence in the north of Africa\'s most populous nation and largest oil producer.