Brig. Gen. Alan Arrojado said the militants identified the slain hostage as Rodolfo Buligao in a note they left on his decapitated body, which was found on a roadside in Maimbung town in Sulu province late Tuesday.
The militants led by Abu Sayyaf commander Yassir Igasan, who killed Buligao, have threatened to behead two coast guard personnel who were kidnapped with Buligao in May.
"They have threatened to kill the two other captives anytime," Arrojado said.
The Abu Sayyaf, with about 400 gunmen split into a few factions, is on a U.S. list of terrorist organizations for deadly bombings, kidnappings for ransom and beheadings.
It is one of at least four small armed groups outside of a peace deal the government signed last year with the largest rebel group, the 11,000-strong Moro Islamic Liberation Front, in an attempt to peacefully settle a decades-long separatist insurrection by minority Muslims in the south of the large Roman Catholic country.
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