BEIJING (AP) — The head of the body that oversees China's major state-owned companies was fired Tuesday in a spreading corruption investigation that appears to be an effort by the country's new leaders to tighten control over government industries. The Communist Party dismissed Jiang Jiemin from his positions as director and deputy party chief of the Cabinet's commission that runs state companies, the official Xinhua News Agency said Tuesday, two days after officials announced an investigation into Jiang. Jiang formerly was chairman of the state-run China National Petroleum Corp., which has been the target of a sweeping graft investigation that also has netted four top executives in recent days. They include three executives at CNPC's listed subsidiary PetroChina. As a full member of the party's Central Committee, which is made up of its top 200 members, Jiang is the most senior official to fall since new leader Xi Jinping took power in November. Xi has made fighting widespread graft a key campaign of his leadership so far, with promises to target both junior and high-level officials. The probe suggests that Xi and Premier Li Keqiang are making a stab at tackling the powerful state-owned industries and their allies who reformers say have hamstrung the government. "The state conglomerates, they are empires unto themselves. They have too much money and too much power," said Willy Lam, an expert on party politics at the Chinese University of Hong Kong. "This could be a signal that the Xi Jinping and Li Keqiang leadership is finally trying to discipline them and keep a tighter rein over the conglomerates." Unconfirmed news reports have also earlier said China's leaders have taken the unusual step of endorsing a corruption investigation into Zhou Yongkang, a former security czar and CNPC general manager with a power base in the oil industry. The probe into China's oil giant also has extended to one of the energy industry's richest self-made entrepreneurs, Hua Bangsong, chairman of Shanghai-based Wison Engineering Services Co., which is a PetroChina supplier. Wison said that Hua, a billionaire, was "assisting the relevant authorities" in their investigations. The company, which provides engineering and construction services to the oil industry, said that its board of directors "is convinced that factual circumstances will be clarified." Wison tried to distance itself from PetroChina, which it counts as one of its major customers, after recent press reports on their business relationship. It said PetroChina first-half contract revenue was insignificant. Wison "operates legally and completed the necessary compliance check" required to list its shares on the Hong Kong stock exchange, the company said in a statement to the exchange posted late Monday. Hua, who founded Wison in 1997, is worth $1.2 billion, according to Forbes. ___ Associated Press reporter Kelvin Chan contributed to this report from Hong Kong.