Albuera Mayor Rolando Espinosa was killed early Saturday morning inside his cell by a still unknown assailant.
The Philippine National Police (PNP) Regional Director confirmed Espinosa's death, reported by GMA News.
Espinosa had been arrested for drug-related charges.
According to Beltejar, Criminal Investigation and Detection Group (CIDG) was supposed to serve a warrant on Espinosa at the jail.
"It was a warrant application. It was purely CIDG. Walang coordination with PNP Region 8," he said.
Beltejar added that details on shooting were still sketchy.
Mayor Espinosa had surrendered to the PNP last August after his name was included in a list of government officials allegedly involved in illegal durgs. At the time, he claimed to know some 30 lawmakers and police commanders involved in illegal drugs.
He subsequently left police custody and returned to work, but was arrested last October 5 over illegal-drug and weapons charges.
Espinosa is the father Kerwin Espinosa, the alleged drug lord who was arrested in Abu Dhabi last month.