The prime suspect in last week's massacre of at least 57 civilians in southern Philippines Andal Ampatuan Jr., a prominent member of a powerful local clan, has been formally charged in court with 25 counts of murder, local media reported Tuesday.

Chief State Prosecutor Jovencito Zuno said more murder cases were expected to be filed as medico-legal reports on the massacre victims would be coming in, the on-line news network INQUIRER.NET said.

Zuno said the charges were filed at Cotabato regional trial court. Ampatuan Jr., son of three-time Maguindanao Governor Andal Ampatuan Sr. and mayor of Datu Unsay, will service life in prison if convicted.

The massacre occurred last Monday when a six-vehicle election caravan of relatives, supporters of Buluan town vice mayor Esmail Mangudadatu, plus a group of journalists, was ambushed on their way to register the town mayor as the governor candidate in next year's provincial election.

Around 100 militiamen herded the Mangudadatu group to a remote area and shot them at close range with M-16 rifles and machetes. The authorities said 57 bodies have been recovered but local military officials said the death toll could have reached 64.

Esmail's challenge to the Ampatuan family's dominance in the province is cited by local media and political analysts as a main reason prompting the massacre.

The Ampatuans wield a considerably clout in the Muslim Mindanao and own a hundreds-strong private army. The volatile region is notorious for loose arms and a decades-old insurgency war between government troops and Muslim rebels.