The municipal police commander, Chief Inspector Sunny Leoncito, said the government forces had been seeking to arrest two men in a drug case, both of whom were believed to have been killed in the raid. But what was to have been a routine antidrug operation turned into an hourlong gun battle on the outskirts of the town of Matalam.

The militants who survived later surrendered and gave up their firearms, including two sniper rifles, several assault rifles and handguns, as well as a rocket-propelled grenade. However, the government operatives did not find drugs on the fighters.

A MILF spokesman, Vol Al-Haq, said the group would file a “strong protest” with the government over the killings. He said he believed that a brief firefight occurred with the government agents before the MILF fighters identified themselves, and that they might have believed they were being attacked by another armed group.

“After they surrendered, they were massacred,” the spokesman said, adding that the group was demanding an impartial investigation jointly carried out with a third party or a nongovernmental organization.

“We don’t deal in drugs and never have,” he said. “Even before the assumption of Duterte as president, we already had our own program that discourages illegal activities, including drugs.”