MANILA, Philippines—One of 57 victims of an election-linked massacre in Maguindanao made a secret audio recording of the horrifying attack blamed on a local politician, the suspect's bereaved rival said Saturday.

"Police told me they have recovered the recording device," said Buluan Vice-Mayor Esmael Mangudadatu, who lost his wife, two sisters and an aunt in Monday's attack in Maguindanao province.

Mangudadatu said he asked one of his sisters to hide the tape recorder in her sock when his wife and female relatives went to a local election office, accompanied by at least 27 journalists, to nominate him for provincial governor in next year's elections.

"I asked her to turn it on as soon as they left," Mangudadatu told DZMM radio here.

"It has a capacity of 288 hours or 12 days," he said.

He said he had not joined the convoy because of a perceived threat to his life.

There was no independent confirmation of the presence of the recording. Police investigators have said they cannot reveal the details of some of the evidence they have collected.

However Justice Secretary Agnes Devanadera said Friday that it was possible the women victims among the group had all been raped before being killed. Police said the recovered cadavers included those of 22 women.

Mangudadatu's rival for the post, Andal Ampatuan Jr., and more than 100 armed followers stopped the six-vehicle convoy before shooting everyone dead, according to indictments released by the Department of Justice on Friday that charged Ampatuan with murder.

Police later recovered 57 bodies, many of them buried in mass graves beneath a mechanical digger with the name of the suspect's father, Maguindanao Governor Andal Ampatuan Sr., printed on its side.