The Batang City Jail members suffered minor injuries from the explosion-as they were jogging past the compound’s Alternative Learning System building at around 5:55 a.m.
NBP Superintendent Ramon Reyes said they believe the attack was the offshoot of a gang war.
He said the NBP administration was investigating a suspect although Reyes declined to give further details.
Reyes said a closed circuit television camera footage showed the grenade thrower coming from behind the joggers, as they were heading toward Building 11.
The hand grenade hit one of the joggers before bouncing toward concrete benches across the ALS building, where it exploded. The jogging inmates were able to scatter quickly.
Reyes said visiting rights and other privileges have been suspended for the day.
Reyes said Justice Secretary Leila de Lima wanted to know how a grenade found its way into the hand of an inmate.