A scion of a powerful clan suspected in the massacre of 57 people in an election caravan in the southern Philippines turned himself in Thursday amid mounting pressure on the government to crack down on lawlessness and warlords.

The dead from Monday’s massacre include the wife, family and dozens of journalists and supporters of a gubernatorial candidate who wanted to challenge the rival Ampatuan clan, which has ruled Maguindanao province unopposed for years.

Andal Ampatuan Jr., a town mayor who allegedly stopped the convoy with dozens of police and pro-government militiamen, surrendered to Presidential adviser Jesus Dureza in the provincial capital of Shariff Aguak, said military commander Lt. Gen. Raymundo Ferrer.

He boarded a military helicopter to a nearby city, from where he will be flown to the capital, Manila, for investigation, Lt. Gen. Ferrer said.

“The family voluntarily surrendered him and they agreed that he will be investigated,” he said.

Ampatuan’s family denied the allegations of his involvement in the slayings.

The military deployed tanks and truckloads of troops throughout the province under a state of emergency to hunt down the attackers and prevent retaliatory violence from the victims’ clan.

Police and soldiers on Wednesday found 11 more bodies at the site of the attack, bringing the death toll to 57. Six of the bodies were discovered in a large pit buried alongside three vehicles, and five were found in a mass grave a few kilometers off the main highway.

The vehicles — a sedan and two vans — were crushed by a large backhoe that ran over and buried them, investigator Jose Garcia said.

Ampatuan’s surrender followed days of negotiations between his family and Dureza, apparently in a bid to prevent hostilities between the clan’s followers and government forces.

Interior Secretary Ronaldo Puno said he had warned the Ampatuan family they risked a military attack unless they turned over Ampatuan Jr. by midday Thursday.

Lt. Gen. Ferrer said the area around the provincial capital was tense after troops disarmed about 350 pro-government militiamen loyal to the Ampatuans. The militia is meant to act as an auxiliary force to the military and police in fighting rebels and criminals but often serve as a politician’s private army.

The clan, which has ruled the province since 2001, helped President Gloria Macapagal Arroyo and her allies win the 2004 Presidential and 2007 Senatorial elections by delivering crucial votes.

Ms. Arroyo came under intense pressure at home and abroad to seek justice for the victims of the massacre, with U.N. Secretary-General Ban Ki-moon and media and human rights watchdogs voicing their concern over the scale of the killings.

The Vice-Mayor of Buluan township, Ismael Mangudadatu, after receiving death threats, sent his wife and relatives to submit his candidacy on Monday when the convoy, including at least 18 journalists, was ambushed.

Mr. Mangudadatu said four people whom he refused to identify told him Ampatuan Jr. was seen with the gunmen.

Five police officers, including one provincial police chief, were relieved of duty and will be brought to Manila to face the investigation together with Ampatuan Jr., Lt. Gen. Ferrer said.

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