At least 10 witnesses will testify they saw Andal Ampatuan Jr. leading the gunmen, including police officers, who blocked his rival's election caravan moments before the Nov. 23 massacre, prosecutor Al Calica told The Associated Press.
Hours later, troops found the bullet-riddled and hacked bodies near the highway sprawled in the grass and hastily buried in three mass graves by a backhoe together with three vehicles.
Ampatuan turned himself in last week and denied the charges.
He is the scion of a clan allied with President Gloria Macapagal Arroyo that has ruled southern impoverished Maguindanao province unopposed for years. His father, the family's patriarch, and six other family members also are considered suspects but have not been charged.
Prosecutors initially filed 25 murder charges against Ampatuan in southern Cotabato city, whose regional trial court is nearest to the massacre site in Ampatuan township.
The five prosecutors handling the murder case carried two boxloads of evidence and affidavits from witnesses from Manila to Cotabato city aboard two air force helicopters. They are expected to ask the court to try the case in Manila for security reasons.
"The evidence is strong," Calica said, adding that at least 10 witnesses provided written testimonies linking Ampatuan to the killings.
He said three of them were in the convoy carrying journalists and the wife, two sisters, an aunt and several supporters of Ampatuan's rival, Vice Mayor Esmael Mangudadatu of Maguindanao's Buluan township.
Mangudadatu had sent his relatives to file his candidacy papers for governorship. Mangudadatu said Ampatuan had threatened to chop him to pieces if he attempted to challenge the Ampatuan family's ironclad control over the province. So, Mangudadatu sent female family members in the belief they would not be harmed.
Quoting the three witnesses, Calica said they managed to turn their cars from the tail end of the convoy and escaped after shots were fired and the gunmen hurriedly took control of the vans and sport utility vehicles in the caravan.
Police cars were parked along the road as the gunmen led the victims in their vehicles to a remote hilltop where they were butchered, Calica said.
Police said earlier they took into custody six officers, including the Maguindanao provincial police chief and his deputy. Two inspectors among them were allegedly seen during the massacre with Ampatuan, said Erickson Velasquez, head of the criminal investigation division.
Prosecutors said the killings were carefully planned and that more charges will follow. At least one witness alleged that the Ampatuan clan had gathered in the patriarch's mansion in the provincial capital of Shariff Aguak days before to plan the killings, said chief state prosecutor Jovencito Zuno.
The graves were dug in advance and a backhoe positioned to bury the bodies, prosecutors said.
The Ampatuans denied any responsibility in the killings in a rare news conference in Shariff Aguak on Sunday.
In Manila, about 1,000 journalists and activists marched Monday to demand justice for the single worst attack on the media anywhere in the world. Thirty of the victims were journalists or their staff. The protesters hackled Arroyo's spokesman Cerge Remonde when he tried to address them outside the president's office.
The carnage drew worldwide condemnation, including from U.N. Secretary-General Ban Ki-moon, the U.S., Australia and EU governments.
Arroyo has declared a state of emergency in Maguindanao and a neighboring province and ordered troops and police to confiscate unlincensed weapons and restore order. But few think the measures will go far enough in a lawless region notorious for political warlords that has been outside the central government's control for generations.