"We are expecting Mayor Andal Ampatuan Jr to be turned over peacefully to the authorities anytime today," Lieutenant-Colonel Romeo Brawner, a military spokesman, told reporters.
'Willing to co-operate'
Al Jazeera's Marga Ortigas, reporting from the town of Buluan close to the massacre site in Maguindanao province, said Ampatuan had told the authorities he would submit himself to investigators to show that he is "willing to co-operate" with the government.
Ampatuan is a member of a powerful local political family, and the son of the provincial governor who himself is a close political ally of the Philippine president, Gloria Arroyo.
|The massacre has sent shockwaves through the Philippines [EPA]
Andal Ampatuan Sr had been grooming his son to take over as governor in elections due next year.
Monday's massacre occurred after about 100 suspected Ampatuan gunmen allegedly abducted a convoy of aides and relatives of a rival politician, Esmael Mangudadatu, as well as a group of accompanying journalists.
The victims were snatched as they were travelling to file election papers nominating Mangudadatu as a candidate for provincial governor in next year's poll.
According to investigators, the victims were shot at close range, some with their hands tied behind their backs, and dumped or buried in shallow graves on a remote hillside.
Mangudadatu, the gubernatorial candidate, was not himself in the convoy because he had received death threats and said he thought the women he sent in his stead would be safe.
|The dead include at least 20 journalists who were travelling with the convoy [Reuters]
On Tuesday, he pressed government officials to immediately arrest and prosecute those behind the killings.
Mangudadatu said four witnesses in his protection had told him the convoy was stopped by armed men loyal to Ampatuan Jr, to prevent his family from filing election papers.
"It was really planned because they had already dug a huge hole [for the bodies]," he said, adding that there were reports from the area that the militia had been blocking the road for a few days.
Among those killed were at least 20 journalists accompanying the convoy, in what media monitoring groups have labelled as the worst ever single attack on journalists.
The massacre has put intense pressure on the Arroyo government to take decisive action against the Ampatuan clan.
She has vowed an all-out effort to bring those responsible for the killings to justice, saying that no one would be seen to be above the law.
In the wake of the massacre the president declared a state of emergency in Maguindanao and a neighbouring province, ordering hundreds of extra troops to the area.