MANILA, December 2, 2009 (AFP) - Police investigating the massacre of 57 people in the Philippines said Wednesday they had made a breakthrough in the case that linked a gunman to a politician charged with masterminding the killings.

Bullet shells found at the scene of the November 23 murders in the southern Philippines matched the gun of a militia member who was controlled by local mayor Andal Ampatuan Jr., police said.

"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," lead police investigator Ericson Velasquez told DZBB radio.

Ampatuan Jr. is the only person to have been charged with the killings, in which 100 of his gunmen allegedly participated, but Velasquez indicated the new development would help police.

"This is a breakthrough," Velasquez said, referring to the results of the ballistics tests. "We can now prove that the gun recovered from the suspect was used at the crime scene."

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, who, he did not name, was one of two militia members arrested at a police checkpoint near the massacre site in the province of Maguindanao shortly after the killings.

Police allege 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 in next year's elections, as well as a group of journalists.

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

Ampatuan Jr. has been charged with 25 counts of murder so far, although prosecutors have indicated he will face more once all the death certificates have been processed.

Aside from the two unnamed 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, the governor of Maguindanao, and at least seven other members of his powerful Muslim clan are being investigated.