Shortly before his retirement late last year, Zhou Yongkang, the longtime chief of China's domestic security apparatus, visited an office of the state oil company where he had started his ascent in the Communist Party hierarchy. He spoke of his undimmed devotion to the company, China National Petroleum Corporation, and to his fellow oilmen.
"Oil is a word that stays in an oilman's heart for his whole life," Mr. Zhou said in his valedictory.
A year later, senior officials associated with the oil conglomerate, including longtime allies of Mr. Zhou's, are enmeshed in spreading corruption inquiries, presenting China's new leader, Xi Jinping, with one of his biggest tests so far: How far and high is he willing to go to clean up China's political elite?
Since his appointment as top leader last year, Mr. Xi has promoted himself as a firm opponent of "flies and tigers": low- and high-ranking officials engaged in graft and bribetaking. On Sunday, he scored one victory when a court sentenced Bo Xilai, a former Politburo member, to life in prison after declaring him guilty of accepting bribes, embezzling state money and abusing his power.
But Mr. Xi must now decide how much further he is willing to pursue other high-level corruption investigations that could either strengthen his authority or unleash risky instabilities within the political elite.
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Mr. Xi is eager to assert authority over the domestic security agencies and the military, said Ding Xueliang, a professor at the Hong Kong University of Science and Technology who has studied corruption in China.
Mr. Zhou, who retired as a member of the Politburo Standing Committee, a rank that is widely assumed to make him effectively untouchable, will be an important test of Mr. Xi's intentions and authority.
"What's going on can be called shaking the mountain to scare the tiger," said Professor Ding, citing a Chinese expression meaning a show of strength to warn others.
"It's also about Xi consolidating control over the key parts of the system," Professor Ding said. "It says to Zhou Yongkang, 'We are in the process of collecting all the evidence of people close to you, and if you don't keep yourself disciplined, we can do more.' Others will also understand that warning."