MANILA, Philippines - An M57 fragmentation grenade exploded under a multicab in Zamboanga City Sunday evening, injuring 2 people.

Salip Hassan Sabban, president of Phase 2 Homeowners' Association, said the victims -- Rogelio Anding and Mikih Anga, who is a retired master sergeant of the Philippine Army -- were sitting outside their homes near the vehicle when the explostion occurred.

Authorities later discovered another fragmentation grenade at a vacant lot near the blast site Monday morning. Elements of the explosives ordnance division identified the second fragmentation grenade as an anti-personnel MK26.

Officials have yet to determine the motive behind the grenade-throwing incidents.