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 and 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.”