It is the first time since the founding of the People’s Republic of China that an official who has held such high office has been the focus of a formal corruption investigation, and in pressing his antigraft crusade to new levels, Mr. Xi has broken a longstanding taboo. Mr. Zhou was once a member of the Communist Party’s top rung of power, the Politburo Standing Committee, and even retired members of that body have always been spared such scrutiny.
The principal allegations against Mr. Zhou emerged from investigations over the past year into accusations of abuse of power and corruption by officials and oil company executives associated with him. Those inquiries have already encircled his son, Zhou Bin, and other family members, the sources said.
Mr. Xi and other leaders agreed by early December to put the elder Mr. Zhou directly under formal investigation by the party’s commission for rooting out corruption and abuses of power, the sources said. They said a senior official went to Mr. Zhou’s home in central Beijing to inform him about the inquiry, and Mr. Zhou and his wife, Jia Xiaoye, have since been held under constant guard.
The people who gave the account were an official with a state broadcaster, a former province-level party corruption investigator, a lawyer with family connections to the party elite, a businesswoman with similar ties and a businesswoman who is the granddaughter of a late leader. They all spoke on the condition of anonymity, citing the risk of recriminations for discussing sensitive politics.
“It’s not like in the past few months, when he was being secretly investigated and more softly restricted,” the lawyer said. “Now it’s official.”
Mr. Xi has amassed imposing power since taking leadership of the party in November 2012, and appears to be pressing the case to bolster his leverage over possible challengers.
He occupied an extraordinary nexus of state-blessed money and power, even by the standards of Chinese politics. Educated in oil-field exploration, he spent much of his career in the state oil industry and wielded considerable influence over the sector, which expanded rapidly at home and abroad as demand for energy surged with China’s booming economy.
Later, while a member of the Politburo Standing Committee, he oversaw the party’s sprawling security apparatus, with control over the police, prosecutors, courts and the main intelligence service. During his watch, the party leadership stressed “stability maintenance” as vital to its survival, and the domestic security budget expanded to overshadow even the military’s. Mr. Zhou’s grim, rough-hewed features added to his image as a politician not to be trifled with.
In taking on Mr. Zhou, Mr. Xi could jeopardize elite unity if the case falters or ignites dissension among party officials and elders, including the retired president, Jiang Zemin, under whose tenure Mr. Zhou became a minister for land and then a province party secretary.
“On the one hand, this would be such a dramatic change from previous practice, and risks generating pushback,” said Christopher K. Johnson, an expert on Chinese politics at the Center for Strategic and International Studies in Washington. “On the other hand, this is a guy who likes to send messages and who has been consistently defying longstanding regime rules of physics now for some time.”
Until now, the highest-ranking politicians subjected to corruption inquiries were serving members of the Politburo, a rung lower than the Standing Committee in the party hierarchy. They included Bo Xilai, an ally of Mr. Zhou’s who was sentenced to life in prison in September for taking bribes, embezzlement and abuse of power.
It is not yet clear whether Mr. Zhou will be prosecuted and punished; internal party inquiries do not necessarily end in criminal charges, even when culpability is found. The government has not made any public announcement about the case, nor has Mr. Zhou, who like other senior Chinese politicians is inaccessible to reporters. The decision to investigate Mr. Zhou was first reported by overseas Chinese news sites, including Mingjing and Boxun, and later by Reuters.
After Mr. Xi took leadership of the Communist Party, he vowed to take on corruption both low and high in party ranks — both “flies and tigers.”
Mr. Zhou, who turns 71 this month, is undoubtedly a tiger. But his power and reputation for highhanded ruthlessness also brought critics, and he appeared diminished after Mr. Bo was detained last year.