“Zhou Yongkang signifies how far Xi will go to establish his authority,” he said, “and it’s a warning to others not to stand in his way.”Until now, the anticorruption campaign’s biggest target had been Xu Caihou, a retired military commander. The party announced in late June that General Xu, a former member of the Politburo, was under investigation for taking bribes, directly or through his family.

The case against Mr. Zhou may center on the fortunes amassed by his family, often in sectors of the economy that were 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 traceable assets worth about a billion renminbi, or about $160 million; much of that wealth was concentrated in the oil and gas industries, where Mr. Zhou was in a position to shape decisions and promotions.

That estimate was based on publicly available records and a limited assessment of their companies’ value; it did not include real estate or overseas assets, which are more difficult to identify and value.

The investigation could send tremors through the Chinese political landscape because of Mr. Zhou’s seniority, and because Mr. Xi and his colleagues have now broken an unspoken compact among China’s elite that leaders who ascended to the summit of power — the Politburo Standing Committee — would be immune from formal investigations, even in retirement.

Mr. Zhou, 71, retired at the same party congress that elevated Mr. Xi, but he remained a potentially dangerous adversary, with ties to other senior retired figures and to officials who rose under him. Still, earlier graft investigations have shaken his bases of influence — in Sichuan Province in China’s 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.

At the zenith of his power, few Chinese politicians were more intimidating than Mr. Zhou. For the five years that he sat on the Standing Committee, he was also in charge of the party committee overseeing the police, the domestic security forces and the civilian intelligence service, as well as courts, prosecutors and prisons. He proved willing to apply police power with little regard for China’s already weak legal fetters.