About 800 inmates escaped a federal prison holding Muslim extremists in northern Nigeria during a sunset attack by gunmen who are believed to be members of a radical sect, a police official said Wednesday.

The attackers went cell by cell at the prison in Bauchi, breaking open locks and setting fire to part of the prison before escaping during the confusion, said Bauchi state police commissioner Danlami Yar'Adua.

Four people died and six others remain in critical condition.

Yar'Adua said police believe Boko Haram sect followers freed by the attack are hiding in the mountains surrounding the pasturelands of the rural region.

Members of Boko Haram — which means "Western education is sacrilege" in the local Hausa language — rioted and attacked police stations and private homes in July 2009, triggering a violent police crackdown. More than 700 people died. Many of those arrested in the wake of the attacks last were being held at the Bauchi prison pending trial.

Yar'Adua said 36 prisoners had returned to the prison on their own by Wednesday morning, hoping to serve out the remainder of their short sentences.

The city itself remained calm Wednesday, as paramilitary police officers guarded the front of the damaged prison. They refused to allow an Associated Press reporter access the prison grounds. Police and military units added checkpoints along roads heading out of the city in hopes of catching escapees.

Boko Haram has campaigned for the implementation of strict Shariah law. Nigeria, a nation of 150 million people, is divided between the Christian-dominated south and the Muslim-held north. A dozen states across Nigeria's north already have Shariah law in place, though the area remains under the control of secular state governments.

In recent months, rumours about Boko Haram rearming have spread throughout northern Nigeria. A video recording released in late June showed a Boko Haram leader calling for new violence as the one-year anniversary of their attack neared.

Meanwhile, police believe motorcycle-riding members of the sect are killing policemen in the region.