(CNN) -- A key prosecution witness said Wednesday that he heard the accused plot the murders of 57 people in what is considered the worst politically motivated killings in recent Philippine history.
The landmark murder trial started Wednesday in Manila, with the witness testifying that the accused even ordered a backhoe to bury the bodies after the planned massacre.
Witness Lakmudin Saliao was a personal aide to Andal Ampatuan Sr., the former governor of Maguindanao province. Ampatuan's son -- Andal Ampatuan Jr., former mayor of Maguindanao province -- is accused of being the ringleader in the November 2009 massacre in Maguindanao.
There are 196 accused, about 500 witnesses and more than 11,000 murder charges involved in the case, which is expected to take years.
The wife and sister of political candidate Ismael "Toto" Mangudadatu and 30 journalists were among the massacre victims.
Mangudadatu had sent his family members to file paperwork allowing him to run for governor of Maguindanao. Their convoy was ambushed, and their bodies were found in a mass grave.
The Ampatuan family plotted the killings to keep Mangudadatu from challenging Ampatuan Jr. in the May 2010 gubernatorial election, Saliao said. The witness also said the Ampatuans went on to discuss how to cover up the killings, by burying the victims in mass graves just off a highway.
The witness said he later heard the elder Ampatuan tell the younger Ampatuan, "You know what to do."
Ampatuan Jr. allegedly replied, "We will kill them all, so that no one can speak."
The Ampatuans have run Maguindanao for years. The province is part of an autonomous region in predominantly Muslim Mindanao, which was set up in the 1990s to quell armed uprisings by people seeking an independent Muslim homeland in the predominantly Christian Asian nation.
An investigation has revealed that members of the Philippine police and army also were involved in the massacre, said an eight-member commission of the Philippine justice department. Yet dozens of the 196 suspects remain at large, said attorney Harry Roque, who represents the survivors of 14 massacre victims.
"It's the job of the police to arrest them. But we all know the calibre of the police. That is part of the problem," he said, adding that the Ampatuan family still controls Maguindanao. "People who were supposed to protect the people became the murderers."
Journalist Winona Cuevas contributed to this report.