China's oil giant at centre of corruption probe

Sun Weidong, former deputy manager of PetroChina subsidiary Yumen Oilfield Co., under investigation;

Yang Guoling, assistant general manager and senior accountant at Yumen Oilfield Co., indicted for corruption;

Three top executives at CNPC supplier Sichuan Star Cable reportedly disappeared in July 2013 and another either fell or jumped to her death in September.

CNPC-linked companies are not the only energy and resources companies caught up in the crackdown.

The former chairman and another former executive of China Resources Holdings Co. (233rd on the Fortune 500 list), are under investigation, as are several top executives at various geology and fuel companies. Zhu Changlin, head of North China operations at the State Grid (7th on the Fortune list), is under investigation, along with several managers at regional power operators.

Jiang and several others of those named have been linked to Zhou Yongkang, one of China's most powerful men until Xi's advent, which has led some observers to suspect the net is tightening around Zhou, a former CNPC chairman and national security chief, who dropped out of sight last year.

The corruption probes into former CNPC deputy general manager Li Hualin and Ji Wenlin, a former regional vice-governor who reportedly cut deals with CNPC, "are among several believed to be part of an unprecedented probe into Zhou," the South China Morning Post wrote earlier this year.

If – and this is a big if – Zhou is formally charged, he would be one of the most senior Communist Party higher-ups ever prosecuted since the Communists came to power in 1949.

In a related story, the South China Morning Post described how Zhou used the CNPC as a springboard to political power in the 1990s, which eventually saw him appointed to the supreme Politburo Standing Committee. There, he chaired the Political and Legal Affairs Committee (PLAC), which controlled police and security forces and the judiciary until some of its functions were transferred to the new State Security Committee, which was set up by the anti-corruption crusading Xi.

Zhou retired in 2012, but the activities that reportedly amassed billions for him and his family apparently came under scrutiny upon Xi's accession to the presidency in 2013.

Apart from rumors, nothing has been heard of him for months, and his name is taboo in official media, as BBC China editor Carrie Gracie noted on her blog.

Some sources claim he is under house arrest. But no one knows for sure, and those that do aren't talking.