In President Xi Jinping’s most audacious move yet to impose his authority by targeting elite corruption, the Communist Party on Tuesday announced an investigation of Zhou Yongkang, the retired former head of domestic security who accumulated vast power while his family accumulated vast wealth.

Mr. Zhou, who retired from the Politburo Standing Committee in late 2012, is the most senior party figure ever to face a formal graft inquiry. Until now, no standing or retired member of the Standing Committee has faced a formal investigation by the party’s anticorruption agency.

The party leadership has “decided to establish an investigation of Zhou Yongkang for grave violations of discipline,” Xinhua, the state-run news agency, reported Tuesday, citing a decision by the party’s anticorruption agency. The terse announcement gave no details of the charges against Mr. Zhou.

Until now, the detention and investigation of Mr. Zhou has been secretive and unconfirmed by the government, although known among party insiders and reported abroad.

Charges against Mr. Zhou could well center on the fortunes made by members of his family, often in sectors once under his sway. An investigation by The New York Times showed that Mr. Zhou’s son, a sister-in-law and his son’s mother-in-law held assets worth some 1 billion renminbi, or about $160 million, much of it in the oil and gas sector that was Mr. Zhou’s political fiefdom, where he could shape decisions and promotions. That estimate was based on publicly available records and a limited assessment of their companies’ value and did not include real estate or overseas assets, which are more difficult to identify and assess.

Mr. Zhou, 71, retired from the party leadership in November 2012, at the same congress that appointed Mr. Xi the top leader, but he remained a potentially dangerous adversary, with ties to more senior retired figures.

Graft investigations have already shaken his bases of influence in Sichuan Province in the southwest; in the nation’s biggest oil and gas conglomerate, the China National Petroleum Corporation; and in the country’s police and civilian intelligence services. Mr. Zhou passed through all these areas during his career, and used his influence to install protégés !!!