Mr Azhar, 56, was declared a suspect after several hours of questioning
Indonesian police have detained the country's top anti-corruption official as a suspect in the murder of a prominent business executive.
Antasari Azhar was questioned on Monday as a witness, but police told reporters that based on strong evidence he would now be interrogated as a suspect.
Nasrudin Zulkarnaen, a former director of a state-owned business, was killed in a drive-by shooting in March.
Analysts say the scandal is a huge blow to the Corruption Eradication Agency.
The independent agency, known as the KPK, was set up in 2003 to lead a huge push by President Susilo Bambang Yudhoyono in tackling corruption.
Mr Antasari has spearheaded the crackdown, including a series of investigations into government officials and institutions.
Mr Antasari, 56, was declared a suspect after several hours of questioning at a Jakarta police station on Monday. Police said that Mr Antasari was among nine suspects in detention.
"Today he was interviewed as a witness and after the end of that interrogation, based on strong evidence, he will be interrogated in this investigation as a suspect," said Jakarta's police chief Wahyono, who goes by one name.
Speaking on Sunday, Mr Antasari said he could not comment on why he had been implicated in the case but said he would "respect the legal process".
He said he was taking leave of absence to deal with the allegations.
Despite recent gains, Indonesia has one of the worst corruption ratings in the world, according to Transparency International.