BEIJING (AP) — China's ruling Communist Party announced an investigation into a feared ex-security chief, demonstrating President Xi Jinping's firm grip on power and breaking a longstanding taboo against publicly targeting the country's topmost leaders.

If he goes to trial, Zhou Yongkang would be 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 mercilessly persecuted political opponents during the chaotic 1966-76 Cultural Revolution.

Until his retirement in 2012, the square-jawed, granite-faced Zhou was one of nine leaders in the party's ruling inner circle — the Politburo Standing Committee — whose incumbent and retired members had been considered off-limits for prosecution in an unwritten rule aimed at preserving party unity.

However, Xi, who is party leader as well as president, has 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.

The party's anti-graft watchdog, the Central Commission for Discipline Inspection, said on its website Tuesday that it is investigating Zhou, 71, for serious violations of party discipline. Although it gave no details, such an announcement typically paves the way for the official to be ousted from the party and face prosecution.

A commentary by official Chinese news agency Xinhua on Wednesday said the investigation "clearly terminated a myth among many people that senior leaders are regarded to be immune from the party discipline regulation and the country's law enforcement."

The announcement ended months of speculation over Zhou amid reports of his family amassing great wealth as authorities began investigating dozens of his associates including several high-ranking officials and businesspeople. One after another, the associates disappeared into the custody of party investigators, foreshadowing problems for Zhou.

On Tuesday, the Chinese news magazine Caijing reported that Zhou's son was arrested by prosecutors in the city of Yichang in Hubei province, accused of "illegal business operations."

Zhou himself was last seen in October and is believed to have been detained sometime thereafter, although there was no public announcement.

By targeting Zhou, in charge of China's massive domestic security apparatus before his retirement, Xi showed the considerable power he has amassed since he took the helm of the party in November 2012.

Tuesday's announcement was a "powerful demonstration" that Xi and his graft-fighting right-hand man, fellow Standing Committee member Wang Qishan, are "really in control," said Ding Xueliang, an expert on Chinese politics at the Hong Kong University of Science and Technology.

"This is a huge, huge success for them. It is really remarkable," Ding said. "For the broadly defined party state system, which has many millions of members, now they have to face the new reality. That is: You are not immune to punishment."

Li Datong, a political commentator who was removed from a state media senior editing job for broaching sensitive subjects, said Xi means to show that "if he's willing to go after a person like Zhou Yongkang, then there's no one he won't be willing to take on."

Although retired, Zhou still had threatened to interfere with Xi's political agenda to protect his personal interests, Li said. "Zhou was a rival who couldn't be entirely controlled."

By dismantling Zhou's spheres of influence, Xi also has freed up important positions in strategic areas of the government, security apparatus and state enterprises that he can fill with his own allies.

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.

Zhou 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. He became a Politburo Standing Committee member and the national security chief in 2007.

To launch an investigation into Zhou, Xi would likely have had to overcome opposition from high-level party officials and retired leaders concerned about how it would hurt the party's image. The move now raises questions of whether more top leaders will be implicated.

"No doubt, there must have been huge resistance" within the party leadership, said Zhang Lifan, an independent expert in Beijing on elite Chinese politics. Referring to Xi's pledge to target senior officials, nicknamed "tigers," in the crackdown, Zhang said, "today, the side that is striking out at the tigers has won the momentum."

The Xinhua commentary Wednesday warned that "as long as the problem of officials' misconduct exists, the drive to hunt both high-ranking 'tigers' and low-ranking 'flies' will never end."

Zhang said now that the case has come to light, the party leadership cannot afford to appear weak or it would risk reprisals from those who opposed the move.

"One thing is for sure, that is the tiger must be beaten to death, otherwise those who attack tigers will lose their sense of security," Zhang said.

Xi has made his drive to clean up the party the hallmark of his leadership. The Xinhua report said that according to the central discipline commission, about 40 officials at provincial and ministerial levels or higher have been investigated for corruption or other serious disciplinary violations since November 2012.

On top of the more than 50 executives of state companies investigated since Xi took charge, it marks the biggest purge of its kind in decades.

Among them, those believed linked to Zhou's case include the former chairman of China National Petroleum Corp., a deputy general manager there, a provincial deputy party boss, a provincial deputy governor and a deputy public security minister.

No specific details were released on the allegations against Zhou. But reports by Caixin, Caijing and The Beijing News, have detailed how Zhou's son, Zhou Bin, built a business empire in oil and real estate through connections that were not explicitly stated but that clearly hinted at Zhou.

Zhou Bin also was business partners with Liu Han, a former multi-millionaire mining tycoon who was sentenced to death in May on charges of running a criminal gang.

Several people on the streets of Beijing welcomed the news, saying it showed the government was serious about rooting out official corruption.

"We feel that the new central leadership has the determination to fight against the corruption and we have seen big moves in this regard," said architect Yang Hao. "We are encouraged by that."

Still, moves against high-level officials are often seen as politically motivated swipes at rivals. Political experts have said Xi's leadership might be better served by moving away from such campaigns and improving institutional checks on power.

"They should move toward building a stronger legal, political, institutional base," Cheng Li, an expert on elite Chinese politics at the U.S. think tank the Brookings Institution, said in a recent interview. "I think that Xi and Wang Qishan will continue this kind of anti-corruption campaign, but if it goes on and touches on too many other leaders, it could be very, very dangerous."

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Associated Press writers Christopher Bodeen and Ian Mader contributed to this report.