Story highlights "It's almost certain" Zhou would be put on trial, a longtime political observer says

Zhou had been on the nine-member Politburo Standing Committee

He had been rumored to be under house arrest before Tuesday's announcement

If indicted, he would be the highest-ranking PRC official ever to face corruption charges

After months of intense political rumors, China's ruling Communist Party announced Tuesday an official probe into a retired senior leader for suspected "serious disciplinary violation."

Zhou Yongkang, the former domestic security czar, was placed under investigation in accordance with Party regulations, the Central Commission for Discipline Inspection said in a one-line statement without elaborating.

Before stepping down in late 2012, Zhou, 71, was one of the nine members that formed the Politburo Standing Committee, China's top decision-making body that effectively rules the country of more than 1.3 billion people. Under his watch, the domestic security budget swelled to surpass that of the military in the name of "maintaining stability," as a widening income gap between the rich and the poor as well as growing discontent over official corruption fueled social unrest nationwide.

Amid an intensifying anti-corruption campaign launched by President Xi Jinping, many political analysts and ordinary citizens have noted ties between an increasing number of disgraced officials and Zhou in recent months. Zhou himself had been rumored to be under house arrest before Tuesday's announcement.

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State media have reported official anti-corruption probes into many of Zhou's family members as well as former associates in the domestic security apparatus, state oil industry and southwestern Sichuan province -- three places Zhou once ruled. Three of his former senior aides were arrested early this month. If indicted, Zhou would become the highest-ranking official ever to face corruption charges in the history of the People's Republic.

"It's almost certain the he would be put on trial and appear in public," said Professor Willy Lam with Chinese University of Hong Kong, a longtime commentator on Chinese politics. "The important thing is that Xi Jinping has proven he's powerful enough to break this taboo of never incriminating former Politburo Standing Committee members -- and in the future he can use this anti-corruption card to thrash his political enemies."

The news on Zhou came on the heels of the downfall of several former high-ranking officials, including a retired top general of the 2 million-strong People's Liberation Army.

Gen. Xu Caihou, a former vice chairman of the Central Military Commission, which runs the world's largest standing army, was expelled from the Communist Party and handed over to prosecutors after being found to have accepted bribes, state-run Xinhua news agency reported early this month. Xu was also a member of the Politburo before retiring in 2012.

State media have characterized Xu as a big "military tiger" caught in the massive anti-graft campaign spearheaded by Xi, who is also the commander-in-chief. Xi banned official extravagance -- from banquets to year-end gifts -- and vowed to target "tigers and flies" alike in his fight against corruption. He resolved to spare no one, regardless of position. CCTV recently touted the capture of 35 "tigers" since Xi took power less than two years ago.

Some 182,000 officials were disciplined in 2013, while courts nationwide tried 23,000 corruption cases, according to the Communist Party's disciplinary commission. State media have cited the trial and conviction last year of former high-flying politician Bo Xilai -- a protégé of Zhou -- as a prime example of Xi's determination to clean up the party, though Bo supporters called the case against him politically motivated.

"Now everybody is really scared -- and this would have a big impact on the behavior of senior Party members," said Lam, the political analyst. "But nobody expects corruption to be eradicated. It is built into the system, a system without checks and balances."