China’s ruling Communist Party announced an investigation into a feared ex-security chief on Tuesday, demonstrating President Xi Jinping’s firm grip on power and breaking a long-standing 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.