AMPATUAN, Philippines (AP) - Philippine authorities, under intense public pressure to make arrests in the country's worst election massacre, said Wednesday they are investigating a member of a powerful clan allied with the government along with four police commanders.
Officials recovered six more bodies Wednesday, bringing the death toll in Monday's attack on an election caravan to 52, including 18 journalists.
No suspects have been formally named in the killings, which provoked outrage beyond the Philippines, with U.N. Secretary-General Ban Ki-moon and media and human rights watchdogs calling on President Gloria Macapagal Arroyo to swiftly punish the attackers.
Arroyo vowed justice for the victims and declared a national day of mourning for them.
"This is a supreme act of inhumanity that is a blight on our nation," she said in a statement. "The perpetrators will not escape justice. The law will haunt them until they are caught."
The bodies were found sprawled a few miles (kilometers) off the main highway in southern Maguindanao province on a remote hilltop, and in two nearby mass graves. Two entire vehicles - a sedan and a van - were also buried in one of the graves, regional police Commander Josefino Cataluna said.
The vehicles were crushed by a large backhoe that ran over and buried them, crime investigator Jose Garcia said.
The dead included the family of gubernatorial candidate Ismael Mangudadatu and 18 Filipino journalists who were accompanying the caravan to file his election papers - the highest number of reporters killed in a single attack anywhere in the world, according to media groups.
Mangudadatu, after receiving death threats, sent his wife and relatives instead. He wanted to challenge a candidate from a rival clan, Andal Ampatuan Jr., whose family has ruled the province unopposed since 2001 with an iron fist backed up by private armies and legions of bodyguards.
Mangudadatu said four witnesses in his protection, whom he refused to identify, told him the convoy was stopped by dozens of gunmen loyal to Ampatuan, currently a town mayor. Mangudadatu's wife, two sisters and other relatives were killed.
Police said they are investigating reports that Ampatuan, four police commanders including the provincial police chief, and dozens of police and pro-government militiamen were among the gunmen who blocked the convoy.
National police Chief Jesus Versoza said the four commanders were relieved of duty and confined to camp while being investigated.
Pro-government militiamen are meant to act as an auxiliary force mobilized by the police or military to fight rebels and criminals, but often act as private enforcers of local warlords.
Arroyo declared a state of emergency in Maguindanao and a neighboring southern province, sending extra troops and police to try to impose the rule of law. Troops set up checkpoints to confiscate illegal weapons, military spokesman Lt. Col. Romeo Brawner said.
Few think Arroyo will be successful in the impoverished, lawless region that has been outside the central government's reach for generations, and where warlords backed by private armies go by their own rules.
Maguindanao's acting governor is Sajid Ampatuan, another son of former Gov. Andal Ampatuan Sr., the clan's patriarch. Members of the family could not be reached for comment.
The clan helped deliver votes for the Arroyo administration in 2004 elections. Human Rights Watch expressed concern Wednesday that the administration's relationship with the family would hinder an impartial investigation.
Arroyo's peace adviser, Jesus Dureza, said he met Tuesday with Andal Ampatuan Sr. and received assurances that his family would cooperate in the probe.
"I have run out of words to describe this. It's ignominious, bestial," Leila de Lima, chairwoman of the independent Commission on Human Rights, said of Monday's attack.
She said there was "strong circumstantial evidence" implicating the Ampatuans.
"I am sure the president must be agonizing now ... but it's a great test of political will, how far the national authorities can go in terms of swift and decisive action. Anything less than that, well, people would be able to put them to task," she said.
Ban, the U.N. chief, condemned the "heinous crime committed in the context of a local election campaign" and hoped that "no effort will be spared to bring justice and to hold the perpetrators accountable," U.N. spokeswoman Michele Montas said.