Gunmen broke into a prison in the central Nigerian city of Minna, freeing around 200 inmates, a police spokesman and security sources said.
The sources said the gunmen stormed the facility late on Saturday in Niger state, near to the capital Abuja. Islamist sect Boko Haram and al Qaeda-linked group Ansaru have been behind several prison raids in recent years, although it was unclear whether they had anything to do with this one.
The police spokesman for Niger state said the breakout was still being investigated.
"It's unclear if this is Boko Haram or some criminal gangs," one of the security sources said. "I doubt there were many high profile Boko Haram suspects being held in Minna."
In central Kogi state last month, gunmen used explosives to blow their way into a prison in the city of Lokoja, killing one person and freeing 144 inmates.
Suleiman Muhammad, a resident of Minna living near the prison, said he heard gunshots but no explosions and saw the guards fleeing shortly after the first assault.
Boko Haram has spread insecurity across north and central Nigeria, setting off bombs, attacking towns and killing thousands of people.
The Council for Foreign Relations (CFR) this week estimated that some 10,340 violent deaths between November 2013 and November 2014 were linked to Boko Haram-related violence.
There are also criminal gangs made up largely of Fulani cattle rustlers in central Nigeria that have killed many hundreds over the past two years.