They said the move was also needed to try and stop the Ampatuan, a powerful opposition group linked to the killings from sabotaging the investigation.

Senior Superintendent Bienvenido Latag, the region's police chief, said: "We cannot dismiss the possibility that they [the Ampatuan clan] may have relatives among those assigned there.

"We want to make sure that our investigation will not be sabotaged from the inside," he said.

Election massacre

At least 57 people were shot dead on November 23 in a remote farming area of Maguindanao in a massacre that shocked the nation.

Andal Ampatuan faces 25 counts of murder, along with other family members [AFP}
Andal Ampatuan Jnr, a mayor of the southern province, has been charged with 25 counts of murder over the massacre, and authorities have said at least eight of his family members are also being investigated.

One of them is Andal Ampatuan Snr, the governor of Maguindanao province who for years had his own private army and enjoyed the backing of Gloria Arroyo, the president of the Philippines.

Police allege Ampatuan Jnr and 100 of his gunmen 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.

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

Replacements have already been dispatched to three towns where about 60 officers were detained for their possible involvement in the massacre.

"Those who were implicated were already placed under the custody of the Criminal Investigation and Detection Group," Espina said.

"The order... is for the purpose of an impartial conduct of the investigation regarding the incident," Espina said.

Investigation findings

Meanwhile, the government pledged to provide security to judges as well as government lawyers handling the Ampatuan case amid reports that some of them had been threatened.

Ronaldo Puno, the interior secretary, said: "We assure them that they will be properly protected and this is already being addressed by the police authorities in the area."

Police also said on Wednesday that they had identified one of the guns used in the massacre – a development they expected would push forward the case.

The assault rifle of a member of the rebel group led by Ampatuan matched some of the shell casings recovered by investigators at the crime scene, a police statement said.

Esmail Canapia, the group's member, and a colleague were arrested at a nearby police checkpoint shortly after the shootings, police said.

"This is a breakthrough," Ericson Velasquez, a police investigator presiding over the case, said. "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 said.