BEIJING: China's Communist leaders on Tuesday decided to place former security czar Zhou Yongkhang under investigation for suspected "serious disciplinary violation."

Sources said the move signals the unraveling of one of the biggest bribery scandal in recent years.

The formal announcement of investigation which comes after months of rumors that Zhou (71) was under house arrest shows investigators and prosecutors are confident of obtaining court verdict against him, sources said.

Zhou was the ninth-ranking member of nine-membered Politburo Standing Committee (PSC) of the Communist Party of China headed by former party leader and ex-President Hu Jintao who handed over power to Xi last year.

Zhou towered over the country's internal security apparatus which has a budget of $110 billion for five years until his retirement in 2012. He was also regarded as an 'oil man' after spending decades building China's state owned energy companies.

There is a strong chance Zhou will eventually be placed under formal arrest before launching investigation. If that happens, it will be the highest point in Chinese president Xi Jinping's 16-month long campaign to root out corruption in public life. Human rights activists have often said he was the most feared politician during his term as security chief as he encouraged the police to adopt strong arm methods.

Xi is also believed to have broken an unwritten rule of not investigating a former member of PSC which Zhou is. He had promised to take down both "tigers" and "flies" in the corruption circuit.

Another powerful politician Bo Xilai who has been jailed was a member of the party's central committee.

Investigators have proceeded with the case with great caution questioning about 300 family members and associates of Zhou before building a strong case against him, sources said. A large number of them have been taken into custody and face punishment.

Chinese authorities are believed to have seized assets worth at least 90 billion Yuan ($14.56 billion) from Zhou's family members and associates.