Two more associates of retired security tsar Zhou Yongkang have been placed under investigation, suggesting authorities are still collecting evidence in their case against him. Zhao Miao, head of the Communist Party's organisation department in Chengdu, Sichuan, was suspected of "serious violations of discipline" - a phrase associated with corruption - the Central Commission for Discipline Inspection announced yesterday. The department is in charge of appointing cadres in the city. Yan Cunzhang, manager of the Foreign Co-operation Department of China National Petroleum Corporation (CNPC), was also taken away by the graft watchdog last week, according to website Caixin.com . The two are the latest in a string of officials - all with ties to Zhou - to come under investigation as part of a crackdown on corruption that is expected to ultimately target the retired leader. Zhao, 55, was taken away by discipline supervisors on Thursday and authorities were forced to hastily cancel a public event he was scheduled to attend the next day, the 21st Century Business Herald reported. Zhang Lifan, a political analyst in Beijing, said Zhao's detention indicated authorities were still building a case against Zhou. "It's very likely that the investigation against Zhou is still inadequate, or is facing obstruction," he said. "Why hasn't his case been made public after so long? A power struggle might be one reason, and the other could be insufficient evidence." Zhang said last year's trial of ex-Chongqing party boss Bo Xilai - whom Zhou supported - showed the prosecution needed to present an airtight case, even if the verdict was a foregone conclusion. "We saw that the evidence presented at Bo's trial was not solid," he said. At least eight leading figures and officials in the state oil sector have been placed under investigation in the past year, including former CNPC president Jiang Jiemin, and Guo Yongxiang, a former deputy Sichuan governor who worked at the Shengli oilfield, the country's second-largest. Zhou headed CNPC, rose to the Ministry of Land and Resources and was then transferred to Sichuan as its party boss. He later served as public security minister before being promoted to the elite Politburo Standing Committee, becoming one of China's nine most powerful men, in 2007. He retired in the leadership transition in 2012. He would be the highest-ranking Chinese leader to face charges in decades. Zhang Min, a Beijing-based political commentator, said the leadership was probably trying to charge Zhou only with corruption, although his case encompassed much more. "To make it a pure corruption case, they need many people and details" to present in court, he said.