"The central authorities may want to investigate the cases linked to Zhou under the code of rule of law, but in practice, the junior-level officials and investigators … handle the cases as political tasks assigned by their superiors," a source said.

They were more concerned with ensuring the suspects were found guilty than ensuring a fair process, the source added.

The questioning, which began nearly two years ago, has been handled by the party's anti-graft watchdog, the Central Commission for Discipline Inspection. Sources say one common opening line in such interrogations is: "You have sided with the wrong [political] camp, so tell me what you know."

If the suspects refuse to provide leads on their associates or mentors, the investigators threaten to take their family members into custody.

Such intimidation tactics had been used in the investigations of several senior provincial officials and at least one senior state-enterprise executive now being investigated, sources said.

One of the more problematic cases is that of Liu Han , the Sichuan mining tycoon with links to Zhou's son, Zou Bin. During the trial, Liu's brother said investigators told him that he had to testify in order to save his other relatives.

"The police beat me every day when I was detained in Beijing," a person who witnessed the trial quoted the brother, Liu Wei, as saying. "They said if I didn't make the confessions, they would arrest my wife and child."

Liu Wei said the investigators tortured him by rubbing a "medical lotion" on his body to force a confession.

The anti-graft agency also appears to be selective about which officials to pursue. In the case of Liu Han, most of his associates remain unaffected by the inquiry. But investigators had arrested Tan Li , a former Hainan deputy governor who was believed to have received about 1 million yuan (HK$1.26 million) in gifts from Liu, including thousands of yuan for his daughter's wedding, sources said.

"No one really knows why Tan Li was the only senior official detained while others were not charged," said one of the sources familiar with Liu Han's case. "If the investigation is being conducted under the rule of law, then everyone who received money from Liu should be punished. But now everyone involved is living in fear in the absence of legal standards and transparency."

More than 150 party officials oversaw Liu's trial, including the vice-president of the country's top court, Li Shaoping .

"It is impossible for the judges to make independent rulings when so many senior officials supervise the case," said the source. "This might have set a very bad example for future cases linked to Zhou Yongkang, as well as the rule of law."

The party plans to use "the rule of law" as a theme for the party plenum, and to use the cases of Zhou and his associates to show that China has achieved more transparency and fairness in its handling of corruption and judicial proceedings.

Li Xiaolin, a prominent lawyer, said it was important to give President Xi Jinping's anti-corruption campaign legitimacy.

"To use inappropriate means in fighting corruption could possibly generate new corruption," Li said at a forum organised by and the Weiheng law firm in Beijing on August 13. "It would also hurt the legal system and damage the reputation of the current anti-corruption campaign."

Implementing the rule of law required that every case strictly adhered to set procedures, said Zhang Yansheng, another lawyer at the forum.

"We have to take a broader view of the anti-corruption campaign and respect the legal process," Zhang said. "The detained officials must also be treated equally under the rule of law."