A senior official linked to Chinese former security chief Zhou Yongkang is being investigated, state media say.
Ji Wenlin, the vice-governor of Hainan province, was being investigated for "suspected serious violation of discipline and laws", the Communist Party disciplinary body said.
No other details were given of the probe, which comes amid a crackdown on corruption by President Xi Jinping.
There are ongoing rumours that Mr Zhou is under investigation.
Zhou Yongkang was until 2012 a member of the Communist Party's top decision-making body, the Politburo Standing Committee, with oversight of China's security apparatus.
There has been no official comment on any investigation, but in recent months several business and political associates of Mr Zhou have come under scrutiny.
Mr Ji worked with Mr Zhou when he was land resources minister. Mr Ji was also an aide to Mr Zhou when he was party secretary in Sichuan province.
He then served as deputy director in the general office of the ministry of public security from April 2003 to December 2008, the People's Daily newspaper said. Mr Zhou was the minister of public security from 2002-2007.
Tiger and flies
Unconfirmed reports have been circulating for months that Mr Zhou is under virtual house arrest.
Several other top officials linked to him are facing investigations, including Sichuan officials Li Chuncheng and Li Chongxi, and former energy chief Jiang Jiemin.
Mr Zhou was also the patron of jailed former Chongqing party chief Bo Xilai.
Bo was found guilty of bribery, embezzlement and abuse of power, and sentenced to life imprisonment in September. Bo's supporters, however, believe he is the victim of a political purge.
Xi Jinping has warned that corruption could topple the Communist Party, and launched an anti-corruption campaign he said would target both "tigers and flies" - high and low ranking officials in the government.
If Mr Zhou were charged it would send shockwaves through the ruling elite, who generally are not investigated once they retire, reports the BBC's Martin Patience in Beijing.
It is not clear whether China's leaders will decide to press ahead with the case against Mr Zhou. Any trial would run the risk of embarrassing revelations being made public that could damage the standing of the party, our correspondent adds.