It has been several decades since a Politburo Standing Committee member has been prosecuted by China's judiciary, whether incumbent or retired. The reported investigation into Zhou, 70, may illustrate the new leadership's determination to exempt nobody in its fight against endemic corruption.
But in a political system in which graft is rampant, party-steered corruption probes into senior officials have often carried political overtones. Prosecutions of high-level officials on graft charges are perceived as moves to ostracize those who have been defeated in factional struggles, without publicizing details of infighting that depict party leaders in a state of disunity.
"I think that the anti-corruption effort is just a political weapon used to take down whoever they want to take down," said Zhang Lifan, a Beijing historian and elite politics expert. "Because when there is widespread institutional corruption, anti-graft efforts are not going to clean up the system. They are a means to get rid of political opponents."
Zhang said Zhou's fate appeared to be still unclear because of media reports that the former leader sent a wreath to the memorial of a party veteran earlier this week. "That's a sign that he still has political presence," Zhang said.
Before his retirement last November during a leadership transition, Zhou was the No. 9 ranking official in the Communist Party, after rising through party ranks over 50 years. He spent the early part of his career in the state-run oil sector, eventually becoming general manager of China National Petroleum Corp., the country's biggest oil and gas company.
Later, he headed the ministry of land and resources and served as party chief of Sichuan province before being put in charge of public security in the early 2000s. Zhou wielded enormous influence over the police and paramilitary forces charged with maintaining internal stability at a time of growing unrest, and he was also perceived to exert control over the oil sector through his associates.
But reports in overseas Chinese media said early last year that Zhou was being scrutinized for being an outspoken ally of ousted politician Bo. Zhou was widely believed to be grooming Bo as his successor and supporting Bo's bid for a seat on the Standing Committee.
The Hong Kong newspaper's report follows announcements by Chinese authorities of investigations into four high-level oil sector executives this past week as well as earlier graft probes into officials in southwestern China believed to be closely linked to Zhou.
The Chinese government said Monday that China National Petroleum Corp. deputy general manager Wang Yongchun was being investigated on suspicion of "severe disciplinary violations." Wang is said to be a close associate of Zhou. Three other executives are also being investigated.
Also, political associates of Zhou in the southwestern province of Sichuan, where Zhou served as a party chief between 1999 and 2002, have come under investigation, including Li Chuncheng, former provincial deputy party chief, and Guo Yongxiang, former vice governor.
"One by one, they fell, which clearly showed they were approaching him. The possibility is fairly high that they have come to a resolution" to investigate him, said Chen Ziming, an independent political commentator.
The report comes as Beijing has intensified an anti-corruption campaign aimed at displaying President Xi Jinping's resolve to target "tigers" and "flies" alike.