A prominent member of a powerful clan suspected of involvement in the killing of 57 people in the Philippines has been taken into custody.
Andal Ampatuan Jr, a local mayor, surrendered to the authorities but denied organising the killings.
Troops and police swooped on towns run by the Ampatuan clan, arresting 20 men suspected of links with the killings.
The massacre happened last Monday, when a convoy of vehicles used by a rival politician was ambushed.
The passengers were taken to a remote hill region, shot at close range and their bodies dumped in shallow graves.
Mr Ampatuan Jr - mayor of Datu Unsay town - was taken by helicopter from his hometown in the restive Maguindanao province yesterday morning to a nearby airport. From there, he was flown to the capital Manila for questioning.
'The family voluntarily surrendered him and they agreed that he will be investigated,' military commander Lt Gen. Raymundo Ferrer told reporters.
Police were also reported to have rounded up and disarmed a 200-member paramilitary force in Maguindanao on Thursday.
The Philippines authorities have been coming under increasing pressure to bring the perpetrators of last Monday's attack to justice.
Andal Ampatuan Jr did not give himself up to police. He was taken away by a senior adviser to president Gloria Arroyo.
The Ampatuan clan have been loyal supporters of the president, but since the killings three members of the family have been expelled from her party.
Mr Ampatuan Jr has not been formally charged, but is officially under investigation.
President Arroyo, who earlier declared a national day of mourning, has promised that the gunmen would not escape justice.
Philippine politician Ismael Mangudadatu has claimed it was gunmen loyal to the Ampatuans who ambushed his supporters as they were travelling to register his name for the polls.
Among the dead were Mr Mangudadatu's wife, his two sisters and several key supporters, as well as at least 13 journalists who were travelling with them to witness his registration as an election candidate.
The Ampatuans have effectively been in charge of Maguindanao for decades.
Andal Ampatuan Snr has served in the Philippines Congress and won the governorship of Maguindanao unopposed for several terms.
His son was reportedly planning a similarly unopposed run to replace his father but then Mr Mangudadatu decided to run as well. —BBC