Beijing's move to place two more allies of former security czar Zhou Yongkang under investigation is a likely sign that President Xi Jinping's administration is busy tightening its case against the once powerful leader, analysts said.
Two officials from Zhou's spheres of influence are being investigated for "serious violations of discipline," Chinese media and officials said, using a phrase that is often a euphemism for corruption.
The ruling Chinese Communist Party's Central Commission for Discipline Inspection posted a one-sentence statement on its website at the weekend announcing that it is investigating Zhao Miao, a key party official in the southwestern province of Sichuan where Zhou was party chief from 1999 to 2002.
The same weekend, authorities detained Yan Cunzhang, head of the foreign cooperation department at China's largest oil company, China National Petroleum Company (CNPC), which Zhou headed in the late 1990s, according to the cutting-edge magazine Caixin.
Zhou's political rise was built on significant power bases in China's state-run petroleum industry and in Sichuan.
While China has yet to make any formal announcement on the investigation into his associates, several of Zhou's immediate family, as well as some key political allies and supporters, are known to be under investigation.
"There were rumors that this would be made public in December, but it never has," former Xinhua journalist Yang Jisheng, now deputy editor of the political magazine Yanhuang Chunqiu, said.
Yang said the authorities were likely keeping silent on the investigation while they compiled sufficient evidence in the case.
"This would mean that there will be no likelihood of the case being overturned," he said. "Or maybe it's because any sort of obstacle now would make the case extremely hard [to prosecute]."
Meanwhile, Beijing-based veteran journalist Gao Yu said a recent public appearance by President Xi's predecessor, Hu Jintao, suggests that Xi is now consolidating his power behind an anti-corruption campaign which he launched on taking the reins of the party in November 2012.
Hu's April 9 visit to Hunan University amid close attention from state media would only have been possible with Xi's full knowledge and backing, observers said.
It could also serve as reassurance to members of Hu's own power base in the Communist Youth League, as the probe into Zhou's allies widens, Beijing-based historian Zhang Lifan told Hong Kong's South China Morning Post newspaper at the time.
"Hu Jintao is laying bare his support for Xi Jinping, after having pretty much no political role since March 2013, and having kept a very low profile," Gao said.
"Hu Jintao may have kept a low profile, but there was nothing low profile about this event," she said.
"I read this to mean that Hu Jintao is supportive of the overall political strategy and of ... some of the specifics of the anti-corruption campaign."
Speculation has been mounting for months that the party leadership under President Xi may be getting ready to charge Zhou, 71, a former member of the party's elite Politburo standing committee, with corruption and abuse of power.
Zhou, who was once a political mentor to jailed former Chongqing party chief Bo Xilai, stepped down from his post as Politburo standing committee member and head of the political and legal affairs commission in November 2012, where he wielded huge power, political analysts say.
His post, from which he oversaw the rolling out of a nationwide "stability maintenance" system and was granted a sharp rise in the domestic security budget, has since been downgraded to report to the all-powerful Politburo standing committee.
If Xi's administration, which has vowed to go after high-ranking "tigers" in a graft crackdown begun last year, goes public with the probe, Zhou will become the most senior Chinese politician to be ensnared in a graft scandal in the history of Communist Party rule.
Earlier this month, Reuters reported that the party has seized U.S. $90 billion in assets linked to Zhou, while his relatives confirmed that his second brother and nephew had been detained as part of enquiries.
Some analysts say the probe is a form of political retaliation after Zhou angered Xi and other high-ranking leaders by opposing the ouster of former Chongqing party chief Bo Xilai, who was jailed for life in September for corruption and abuse of power.
Zhu Qindi, wife of Zhou's nephew Zhou Xiaohua, dismissed the charges against the family in an interview with RFA earlier this month, saying her family is being "bullied" by Xi's administration.
Last month, official media mentioned the investigation linked to Zhou for the first time since unconfirmed reports first began to emerge last year.
Reported by Yang Fan for RFA's Mandarin Service. Translated and written in English by Luisetta Mudie.