GENERAL SANTOS CITY – (UPDATE) A firearm seized by authorities from one of two militiamen arrested shortly after the massacre in Maguindanao matched some of the spent shells that were found at the scene of the crime, police confirmed and called the discovery a “breakthrough” Wednesday.

Chief Superintendent Felizardo Serapio, police ground commander in the investigation, identified the civilian volunteer officer (CVO) as Esmael Canapia.

Lead police investigator Ericson Velasquez, who declined to name the suspect, said over dzBB radio that the militia member was controlled by Datu Unsay Mayor Andal Ampatuan Jr.

"Some of the empty shells that we recovered at the scene matched the gun recovered from the CVO [Civilian Volunteer Officer] who was arrested as he left the crime scene," Velasquez said, calling the results of the ballistic tests “a breakthrough.”

"We can now prove that the gun recovered from the suspect was used at the crime scene," said Velasquez.

Serapio said the physical match was established by Scene of Crime Operatives and elements of the police's Crime Investigation and Detection Group (CIDG).

Velasquez said other guns recovered from other members of the militia were also being tested. "Our investigation showed that many guns were used in the commission of the crime," he added.

The investigator said the suspect was one of two militia members arrested at a police checkpoint near the massacre site in the province of Maguindanao shortly after the killings.

Fifty-seven people were killed in what has been considered the worst election-related violence ever recorded in the country.

Ampatuan Jr., son of incumbent Maguindanao Governor Andal Ampatuan Sr., has been detained and charged with 25 counts of murder with the regional trial court in Cotabato City, although prosecutors have indicated that he would face more once all the death certificates have been processed.

Police allege that Ampatuan and his gunmen kidnapped and shot dead the occupants of a convoy that included relatives of his rival for the post of Maguindanao governor, Buluan vice mayor Esmael Mangudadatu, in next year's elections, as well as a group of journalists.

Mangudadatu, said the killings were carried out to stop him from running for office.

Aside from the two militiamen in custody, four policemen who were manning a checkpoint close to the massacre site have also been detained.

Prosectors have said Ampatuan Jr.'s father, and at least seven other members of his powerful Muslim clan were being investigated.