Yenagoa, Nigeria – A separatist group attacked a prison in Nigeria early Monday morning, freeing more than 1,800 prisoners.  The gunmen used explosives, rocket-propelled grenades, and rifles to free the inmates.

Authorities believe the Indigenous People of Biafra (IPOB), a ‘banned’ separatist group, was responsible for the attack. The group has denied involvement.

“The Owerri Custodial Centre in Imo state has been attacked by unknown gunmen and forcefully released a total of 1,844 inmates in custody,” its spokesman said in a statement late on Monday.

Nigeria has faced an Islamist insurgency in the northeast of the country for a decade.  President Muhammadu Buhari said the attack was an “act of terrorism”. He ordered his security forces to re-arrest the prisoners who escaped.