Philippine President Gloria Arroyo Wednesday vowed to hunt down the perpetrators of a political massacre that left 57 people dead, as one of her allies was named the prime suspect. Arroyo faced increasing pressure to take decisive action as more bodies were pulled out of shallow graves and relatives of the victims reported horrifying details of the killings, including that two women shot dead were pregnant. 'This is a supreme act of inhumanity that is a blight on our nation,' Arroyo said in a statement as she declared Wednesday a national day of mourning. 'The perpetrators will not escape justice. The law will hunt them until they are caught. No citizen in our nation should ever have to fear for his or her life in the free expression of political will.' Police earlier said the top suspect in the massacre was Andal Ampatuan Jnr, a member of Arroyo's ruling coalition and the son of a powerful regional politician who has helped secure votes for the president in previous elections. Ampatuan Jnr is a local mayor in Maguindanao province, a lawless part of the southern Philippines where powerful Muslim clans rule vast areas backed by their own private armies and often out of the national government's control. His father of the same name is the provincial governor who commands his own private army and is also a member of the ruling Lakas Kampi CMD coalition. Arroyo attracted criticism by sending a special envoy, Jesus Dureza, to the Ampatuan camp on Tuesday to get the clan to pledge its cooperation in an investigation, rather than to arrest the son. He emerged from that meeting saying the Ampatuans would cooperate, but on Wednesday night Arroyo's chief aide called for the suspects to surrender. 'It definitely would help if, those who feel that they are already being considered as suspects, for them to turn themselves in and cooperate with the law enforcement agencies,' Executive Secretary Eduardo Ermita told reporters. Ermita's plea came after Arroyo imposed emergency rule in Maguindanao and neighbouring Cotabato city, another Ampatuan stronghold, on Tuesday. The massacre occurred after about 100 Ampatuan gunmen allegedly abducted a convoy of aides and relatives of a rival politician, Esmael Mangudadatu, plus a group of journalists. The victims were abducted as they were travelling in a six-vehicle convoy to nominate Mangudadatu as the opposition candidate for provincial governor in elections next year. He was not in the convoy. They were shot at close range, some with their hands tied behind their backs, and dumped or buried in shallow graves on a remote farming road close to a town bearing the Ampatuan name. Ampatuan Snr had been grooming his son to take over as governor of Maguindanao, and the victims' relatives have alleged the Ampatuans organised the murders so that Mangudadatu would not run for that post. Mangudadatu said the body of his murdered wife had been horrifyingly mutilated and that his dead sister and aunt had both been pregnant. 'We can't call him an animal because I have pets and they are tame. No, he is a monster. They are monsters,' Mangudadatu told reporters, referring to Ampatuan Jnr and his gunmen. 'My wife's private parts were slashed four times, after which they fired a bullet into it,' he added. 'They speared both of her eyes, shot both her breasts, cut off her feet, fired into her mouth. I could not begin to describe the manner by which they treated her.' The death toll rose from 46 to 57 after 11 more bodies were pulled out of shallow graves on Wednesday, according to police. The victims included at least 13 local journalists who had been intending to report on Mangudadatu's governorship nomination, making Monday's killings the deadliest single attack on the media in history. The ruling coalition's candidate for next year's presidential election, ex- defence secretary Gilberto Teodoro, said the Arroyo administration must pursue the attackers, 'whether they are political allies or not'. The Philippine Commission on Human Rights chairwoman, Leila De Lima, also called for immediate action by the president, describing Ampatuan Jnr as a 'warlord'. 