Detainees suspected of belonging to the Islamist militant group Boko Haram overpowered guards at the headquarters of Nigeria's secret police in Abuja Sunday, triggering a one-hour gun battle that killed at least 21 people.
Security officials say the incident began when a guard went to give the detainees food. They say one of the suspects attempted to overpower the guard by hitting him over the head with his handcuffs. Other guards responded and shots were fired.
It is not yet known how many of the dead were inmates or security personnel.
Jail breaks by Islamists in Nigeria are common. Two weeks ago, Boko Haram militants attacked the main Nigerian military barracks in the northeast, freeing dozens of prisoners. Most were reported killed trying to escape.
The Boko Haram insurgency has been raging since 2009 and is blamed for thousands of deaths.