Zhou Yongkang, China's top security official, attends a plenary session on the draft amendment to the Criminal Procedure Law as China's National People's Congress (NPC) takes place in Beijing, China, on Thursday, March 8, 2012. China is holding the fifth session of the eleventh NPC this month. Photographer: Nelson Ching/Bloomberg via Getty Images

BEIJING (AP) — Chinese authorities arrested the once-feared ex-security chief Zhou Yongkang and launched a criminal investigation Saturday on charges ranging from adultery and bribery to leaking state secrets, after expelling him from the Communist Party overnight.

The developments, announced shortly after midnight, pave the way for a trial of the most senior figure so far to be ensnared in President Xi Jinping's anti-corruption crackdown and appear to seal the downfall of a formerly powerful politician once considered a potent rival for Xi.

The square-jawed, granite-faced Zhou, 72, is the highest-level official to be prosecuted since the 1981 treason trial of Mao Zedong's wife and other members of the "Gang of Four" who persecuted political opponents during the 1966-76 Cultural Revolution.

Although the case against Zhou has been touted by state media as another example of the party's determination to fight corruption regardless of one's rank, some analysts say it is part of factional politics in the ruling party's uppermost echelon.

"The fundamental issue remains the power struggle," Beijing-based historian and independent political observer Zhang Lifan said.

The announcement past midnight on a weekend is a sign that the party leadership wants to downplay its impact, Zhang said. An allegation that Zhou leaked state secrets may give authorities a reason to close the trial and keep dirty politics under the wraps, he added.

Li Cheng, director of the John L. Thornton China Center at Washington-based think-tank Brookings, said the prosecution against Zhou was a genuine effort by Beijing to root out corruption to rebuild the party's image.

Zhou had been under the party's internal investigation for "severe disciplinary violations" — a phrase is usually used to describe corruption — since last December, a year after he retired as a Standing Committee member of the party's Politburo. Investigations were made public in July.

"He abused his power to help relatives, mistresses and friends make huge profits from operating businesses, resulting in serious losses of state-owned assets," the official Xinhua News Agency said.

The party's Central Commission for Discipline Inspection said in a statement posted online that the decision to expel Zhou was made Friday at a meeting of the 25-member Politburo. Shortly after the expulsion was made public early Saturday, prosecutors announced Zhou's formal arrest and opened a criminal case against him, the party said.

The investigation had found that Zhou had "seriously violated the Party's political, organizational and confidentiality discipline," Xinhua said.

"Zhou leaked the Party's and country's secrets," the Xinhua report went on to say, without revealing what he might have leaked, or to whom. "He seriously violated self-disciplinary regulations and accepted a large amount of money and properties personally and through his family. Zhou committed adultery with a number of women and traded his power for sex and money."

Any trial would be expected to have a foregone conclusion with Zhou's conviction, because the outcomes of such high-profile trials are widely believed to be negotiated among top leaders ahead of time.

Li of Brookings said Zhou would either get life imprisonment or a suspended death sentence. A line in the official report that investigators discovered other, unspecified crimes indicates Beijing could be negotiating for Zhou's cooperation during the trial, Li said.

Former members of the powerful Politburo Standing Committee had long been considered off-limits for prosecution in an unwritten rule aimed at preserving party unity. But Xi vowed to go after both low- and high-level officials in his campaign to purge the party of corruption and other wrongdoing that have undermined its legitimacy in the public eye.

"What Zhou did completely deviated from the Party's nature and mission, and seriously violated Party discipline. His behaviors badly undermined the reputation of the Party, significantly damaged the cause of the Party and the people, and have yielded serious consequences," the report said.

Zhou was once perceived as untouchable, with expansive patronage networks covering the sprawling southwestern province of Sichuan where he was once party boss and controlled the state oil sector, police and courts.

More significantly, as China's security chief, he oversaw the country's domestic spy agencies, a position that afforded him access to information on other high-ranking politicians who might pose a threat to him.

Zhou was born the son of an eel fisherman in a little-known eastern village, the eldest of three boys and the only one to attend university, from which he graduated as an engineer, according to financial news magazine Caixin.

He spent the early part of his career in the oil sector, rising through the ranks over several decades to become the general manager of China National Petroleum Corp., one of the world's biggest energy companies, in 1996.

He then served as party chief of Sichuan province between 1999 and 2002, and became a Politburo Standing Committee member and the national security chief in 2007.

Critics have said policies and practices introduced by Zhou, including indiscriminate crackdowns and harsh measures to preserve social stability have trampled on laws and sabotaged China's fledgling legal system. But it is almost certain Zhou will not be held accountable for such offenses.

"I believe authorities may want to put less political emphasis on Zhou's case and want to handle it as a criminal case," said Hong Daode, professor of criminal justice at China University of Political Science and Law.

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Associated Press researcher Henry Hou contributed to this report.