The announcement is a major break in the probe into the killing of Munir Thalib and represents the first formal acknowledgment by authorities that the powerful State Intelligence Agency may have been involved.
Police spokesman Abubakar Nataprawira said Muchdi Purwoprandjono, a deputy head of the agency at the time of the murder, was arrested late Wednesday after voluntarily turning up for questioning.
He said Purwoprandjono had been declared a suspect in the murder, a formal step in Indonesian criminal investigations, which means police believe they have enough evidence to bring the case to trial.
