Philippine police investigators on Thursday said five women might have been raped before they were killed in the country's worst election-related violence last week while forensic experts found evidence indicating the toll could still rise.

Laboratory tests confirmed the presence of semen on five female bodies unearthed from a mass grave in Maguindanao province where the massacre victims were buried, said Superintendent Ruby Grace Sabino-Diangson, chief of the medico legal division.

Signs of bruises were found around the genital area of one victim's body.

Diangson told a press briefing that initial findings also revealed that the victim was discovered on the crime scene with " her pants pulled down" and that indicated there was sexual penetration before death.

At least 57 people, including journalists, lawyers, innocent bystanders, were killed on Nov. 23 when around 100 gunmen abducted an election caravan in the volatile southern region of Mindanao. The victims were shot dead at close range and were buried amass. The authorities have arrested and charged a local politician -- Datu Unsay Mayor Andal Ampatuan Jr. -- as the prime suspect.

Police on Wednesday recommended murder charges to be filed against at least four other members of the powerful Ampatuan clan. Troops have surrounded the family's sprawling mansions in the capital of Maguindanao province since last Thursday following President Gloria Macapagal-Arroyo imposed the state of emergency in the province, ruled by the Ampatuans for almost a decade.

The abducted convoy was to register the family's political rival -- Buluan town vice mayor Esmail Mangudadatu -- as the governor's candidate to compete with Ampatuan Jr. in next year's election. The exact number of people going with the convoy remains unclear but 57 bodies have been discovered.

An independent forensic team led by Peruvian forensic anthropologist Dr. Jose Pablo Baraybar on Thursday said it is now almost sure that there is a 58th body that is yet unaccounted for after his team's discovery of a partial upper left denture with a metal clasp at the crime scene that did not fit any of the remains already recovered.

"We need a backhoe to empty out the previously excavated mass graves to sift through the debris to ascertain whether an untouched part of the grave may still hold some human remains," said Dr. Baraybar in a statement. "It has to be carefully done."

Media group described the massacre as a "chilling prelude" to the upcoming national elections, in which 17,800 government posts will be decided. Election violence is common in the Philippines. During the mid-term elections in 2007, more than 120 people were killed in election-related crimes.