Security forces raided four compounds on Friday belonging to a powerful clan suspected in the massacre of 57 people in the Philippines' worst political violence, as eight journalists among the victims were buried in a common grave.

The scion of the clan, Andal Ampatuan Jr., turned himself in last week and was charged with multiple counts of murder. His father _ the family's patriarch _ and at least four other clan members also are considered suspects in the Nov. 23 attack on a political rival's convoy but have not yet been charged. They deny any involvement.

Army troops and police, armed with a warrant, stormed four mansions belonging to the Ampatuans on Friday and found firearms and ammunition, local military spokesman Maj. Randolph Cabangbang said.

For several days, hundreds of security forces have surrounded the sprawling compounds in impoverished southern Maguindanao province to prevent the clan from escaping. They dug out an entire arsenal including mortar shells, light machine guns, assault rifles and bazookas buried near the patriarch's mansion, said armed forces spokesman Lt. Col. Romeo Brawner.

Some of the weapons and ammunition were marked as belonging to the national police and the Department of National Defense. Three armored personnel carriers also were seized, Brawner said.

President Gloria Macapagal Arroyo ordered an investigation of police and soldiers who may have supplied the weapons and ammunition, national police Chief Jesus Verzosa said.

Arroyo also ordered the creation of an independent commission to oversee the dismantling of clan-dominated private armies in the country and investigate the carnage, Interior Secretary Ronaldo Puno said.

The 57 people killed in the massacre included the wife, sisters and supporters of Esmael Mangudadatu, who had sent them to submit his candidacy papers for governor, challenging the Ampatuan clan's iron-fisted control of the province.

Thirty journalists and their staff _ the highest number of reporters slain in a single attack anywhere in the world _ were also killed in the attack. Eight of the journalists were buried in a common grave Friday after a funeral procession in which their coffins were carried on a flatbed truck bedecked with flowers and a banner reading "We want justice." Residents threw more flowers as the procession passed.

"It is very difficult to deal with the pain," said Eliver Cablitas, husband of Marites Cablitas, 38, publisher of the tabloid News Focus, who left behind three teenagers.