MANILA, Philippines—(UPDATE) Datu Unsay mayor Andal Ampatuan Jr., the prime suspect in the murders of at least 57 people in Maguindanao, is now being detained at the National Bureau of Investigation in Manila, NBI Director Nestor Mantaring said Thursday evening.

Ampatuan, scion of the influential Ampatuan clan in Mindanao, was flown to Manila to face charges he led a pack of about 100 gunmen Monday who slaughtered family members and supporters of a rival politician.

Multiple murder complaints were being readied by a panel of prosecutors, said Justice Secretary Agnes Devanadera.