Chinese state media praised the Communist Party's decision to open a corruption probe into retired senior politician Zhou Yongkang.
The party announced late Tuesday it is investigating Zhou, China's former security czar, for unspecified "serious disciplinary violation."
The 71-year-old Zhou, who retired in 2012, was a member of the Politburo Standing Committee, China's highest governing body.
The investigation breaks a longstanding, unwritten rule that China's top leaders would be immune from such discipline.
In an editorial, the party-controlled Global Times said the move "reveals the zero-tolerance stance of the Chinese leadership toward corruption."
It said the Chinese public embraced the probe, saying it is now "more convincing" to believe all officials will be subject to the rule of law.
The official Xinhua news agency said the announcement shows the party's determination to "purify itself and run itself with strict discipline."
Since taking power in 2012, President Xi Jinping has carried out a highly publicized graft crackdown, vowing to go after both high-ranking "tigers" and low-ranking "flies."
Zhou is the highest-ranking official yet to be brought down. His downfall was expected, since many of his associates and family members have been targeted in recent months.
He is a close ally of disgraced former Politburo member Bo Xilai, who was sentenced to life in jail last year for taking bribes, embezzlement and abuse of power.
For now, the investigation into Zhou is being handled by the party's internal disciplinary committee.
State media reported Wednesday that it is very likely the committee will expel Zhou from the party and transfer his case to the courts, where defendants have little chance of escaping conviction.