Capt. Jo-ann Petinglay, an army spokeswoman, said Friday that an initial assessment shows the explosion was from a 40 mm round fired from a grenade launcher which landed beside the guard post of a special forces battalion headquarters in southern Cotabato City.
Authorities are still determining the identities of the suspects and a motive behind the last evening blast.
Cotabato is a trading hub 545 miles (880km) south of Manila. It has been hit previously by bombings blamed on Muslim rebels and extortion gangs.