Nov. 25 (Bloomberg) -- Philippine police said the death toll in the nation’s worst act of election-related violence rose to 57 as more bodies were exhumed from a mass grave on the southern island of Mindanao.

Police and the National Bureau of Investigation were to summon several people for questioning today for the Nov. 23 attack on supporters of a local politician in Maguindanao province, President Gloria Arroyo’s office said. Her party expelled three members of a clan linked to the ambush.

The “government is going very seriously on the case,” Eduardo Ermita, Arroyo’s executive secretary, said at a briefing in Manila.

Many of the victims, among them women who were raped and journalists, were found in a mass grave after about 100 gunmen stopped a convoy of people on their way to file the politician’s application to run for governor of Maguindanao, the military said. Arroyo yesterday put Maguindanao and neighboring Sultan Kudarat province under a state of emergency to prevent further violence in the region.

The number of deaths is the highest from a single incident of election-related violence in the nation’s history, according to the Philippine Center for Investigative Journalism.

Elections in the Philippines are often marred by bloodshed, with provincial politicians maintaining private militias. About 126 candidates and supporters were killed in the months leading to the 2007 elections and 186 in 2004, according to the Philippine Daily Inquirer.

Muslim Minority

Mindanao is home to most of the nation’s Muslim minority, some of whom have been fighting a separatist war for decades. It’s also home to the al-Qaeda-linked militant Abu Sayyaf organization, among groups engaged in kidnapping.

At least 12 journalists were among those killed, the most in a single day “in the history of journalism,” Reporters Without Borders said.

The death toll rose from 52 earlier today, after five more bodies were discovered, Chief Superintendent Josefino Cataluna, the Central Mindanao police director, said in a phone interview. Six bodies were recovered earlier, two of them in two vehicles that were also buried, Cataluna said. One of the vehicles had a broadcast network’s markings.

The police will investigate Paisal Umpa, chief police superintendent of the Autonomous Region of Muslim Mindanao, in connection with the killings, national police spokesman Leonardo Espina told reporters in Manila today.

Authorities yesterday suspended and detained Maguindanao police chief Abusana Maguid and three of his officers pending the results of an investigation. Witnesses said the officers were at the scene of the crime, according to Espina. Maguid may bear “command responsibility” for his men, he said.

Extra Troops

Arroyo deployed extra troops and ordered Director General Jesus Versoza, the national police chief, to lead the investigation into the killings. The investigation should be independent, according to New York-based Human Rights Watch.

“Far too many people have been gunned down while President Arroyo has sat on her hands,” Human Rights Watch said in a statement today. “The possible involvement of state forces in the Maguindanao massacre means that security personnel shouldn’t be allowed to interfere in an independent investigation.”

The ambushed convoy was mostly made up of supporters of Buluan City Vice Mayor Esmael Mangudadatu, on their way to file his candidacy for Maguindanao governor next year.

Rape Victims

Mangudadatu, who wasn’t in the convoy, told local media his wife and two sisters were among the dead and that some of the women in the group were raped before they were murdered.

Mangudadatu’s bid may have pitted him against the Ampatuan family, which has controlled Maguindanao since 2001.

Jesus Dureza, Arroyo’s adviser on Mindanao affairs, said he has met with the Mangudadatus and Ampatuans. The families are both allies of Arroyo, Ermita said.

The Ampatuans pledged to cooperate with the investigation, Dureza said in a GMA News TV interview. The executive committee of Arroyo’s political party, Lakas-Kampi, voted unanimously to expel three Ampatuan family members, Gilberto Teodoro, chairman of the party and its candidate to succeed Arroyo, said in a phone interview in Manila.

The committee voted to expel Andal Ampatuan Sr., Zaldy Ampatuan and Andal Ampatuan Jr., the “prime suspect,” Teodoro said. “As titular heads, they had the responsibility to ensure none of their clan members will go astray. We feel they have failed to do that so continued membership in the party is untenable.”

Group Suspected

A group linked to the mayor of Datu Unsay, a town in Maguindanao, was involved in the hijacking, police spokesman Espina said today, citing “initial reports.” He didn’t respond to calls or text messages seeking elaboration.

The mayor of Datu Unsay is an Ampatuan. Staff in the offices of two Maguindanao officials said there are no phones in Datu Unsay, while a mobile phone for another office wasn’t answered.

The Southeast Asian nation will choose a new president and thousands of national and local officials in May. The nation’s Commission on Elections will accept filings for candidacies until Dec. 1.

To contact the reporters on this story: Cecilia Yap in Manila at cyap19@bloomberg.net; Joel Guinto in Manila at jguinto1@bloomberg.net.

Last Updated: November 25, 2009 10:55 EST