An explosion rocked the maximum security compound of the New Bilibid Prison (NBP) Thursday morning, killing one inmate and injuring 19 others.

Bureau of Corrections (BuCor) director Franklin Jesus Bucayu said the grenade explosion occurred at 9:55 a.m..

NBP Superintendent Richard Schwarzkopf Jr. said the grenade, which was initially identified as MK-2 fragmentation, exploded outside dormitory 5-B where members of the Commando Gang are detained.

The fatality, identified as Jojo Sampo, succumbed to injuries at 10:35 a.m. at the penitentiary’s hospital. He is a member of the Commando gang.

Two of the 19 others who were injured in the blast remain in critical condition. Only one among the wounded inmates belongs to another gang, the Batman group.

Bucayu said prison officials are still investigating who was behind the explosion. But according to various sources, the target was high-profile inmate and Commando gang leader JB Sebastian.

Sebastian’s name was not included in the list of casualties in the explosion.

Prison authorities are not discounting the possibility that the blast is related to the series of raids inside NBP where guns, drugs, and other contraband items were uncovered.