BEIJING (AP) — China's ruling Communist Party announced Tuesday it is launching an investigation into former domestic security chief Zhou Yongkang, who was once one of the country's most feared leaders. 
 
 The party's anti-graft watchdog, the Central Commission for Discipline Inspection, said on its website that it is investigating Zhou for serious violations of party discipline, but gave no details. 
 
 Until his retirement in 2012, Zhou was one of nine leaders in the party's ruling inner circle, whose members had until now been considered off-limits for prosecution in an unwritten rule aimed at preserving party unity. 
 
 The announcement ended months of speculation over Zhou's fate that had built up as several high-ranking officials and businesspeople and dozens of other known associates came under investigation. 
 
 One after another, they disappeared into the custody of party investigators, foreshadowing the problems that lay ahead for Zhou. ||||| Media playback is unsupported on your device Media caption The BBC's Damien Grammaticus in Beijing says the decision to investigate sends "significant" messages to both the public and politicians 
 
 China's former security chief Zhou Yongkang is being investigated for suspected "serious disciplinary violation", state media say. 
 
 The news confirms rumours about the hugely powerful former minister, who has not been seen in public for months. 
 
 Mr Zhou headed China's Ministry of Public Security and was a member of the top decision-making body, the Politburo Standing Committee. 
 
 The move to target him will send shockwaves through the political elite. 
 
 With today's public disgrace of such a senior figure, Xi Jinping is signalling that his campaign is different 
 
 He is the most senior Chinese official to be investigated since the Gang of Four - which included the wife of late leader Mao Zedong - in the early 1980s. 
 
 Zhou Yongkang retired in late 2012, as Xi Jinping took over from Hu Jintao as the Communist Party leader and China's president. 
 
 Since the transition, Mr Xi has introduced a wide-ranging crackdown on corruption within the party, warning graft could threaten the organisation's very survival. 
 
 Bo Xilai ally 
 
 In a brief statement, state-run Xinhua news agency said the investigation would be conducted by the Communist Party's corruption watchdog, the Central Commission for Discipline Inspection. 
 
 No timescale was given for the probe, which has been widely expected. 
 
 Several individuals believed to have had close ties to Mr Zhou have also been targeted in corruption investigations in recent months. 
 
 Damian Grammaticas, BBC News, Beijing 
 
 So the tiger-trap has been sprung. What does it mean? In his anti-corruption drive General Secretary Xi Jinping has publicly promised to hunt "tigers and flies", Communist Party officials however powerful or lowly who have lined their own pockets. 
 
 Now he has bagged a tiger. No such senior leader has been brought down by the party in more than 30 years. In truth Mr Zhou was a toothless tiger, retired, increasingly isolated, a spent political force. For months rumours have abounded that the former security chief was being probed. His family, associates and political allies have been caught in the dragnet of corruption investigations. 
 
 But this is about more than tackling corruption. 
 
 With this move Xi Jinping has signalled he feels powerful enough to take down any figure, to neutralise any rival networks competing for power inside the Communist Party. He is asserting his own power and authority. And he is showing he wishes to govern China in the old way, not by opening the party to more outside scrutiny over oversight, but by the use of brute political force, the party policing itself, controlled tightly from the top. 
 
 Mr Zhou's career saw him head both the ministry charged with overseeing domestic security and China's largest energy company, the China National Petroleum Corporation (CNPC). 
 
 He was also the party's top official in Sichuan province. 
 
 Allies of his from all three areas are now the subject of various investigations. 
 
 Only a handful of people serve on the Politburo Standing Committee (in Mr Zhou's time nine, currently the number is seven) and they are seen as the most powerful individuals in China. 
 
 Zhou Yongkang was also an ally of Bo Xilai, the one-time high-flying former Chongqing party chief who was jailed last year. 
 
 In initial reactions observed on Weibo, China's Twitter-like micro-blogging platform, netizens appeared to praise the announcement. 
 
 Thumbs-up emoticons, "Got him!" and "Good job!" were just some of the more popular responses. 
 
 Many also made reference to "tigers", a reference to Mr Xi's promise to include high-ranking officials in his anti-corruption campaign. 
 
 "Now that is what I call an old tiger. Good job Mr Xi," said one user. "This is just one tiger out of many, to catch tigers one must have determination, don't stop now!" said another. 
 
 It was not clear to what extent officials were censoring negative comments, as has been done in the past. ||||| BEIJING—China launched a formal investigation into one of the Communist Party's most senior figures, lifting a cloak of immunity that has shielded the country's highest ranks as President Xi Jinping made his boldest attempt yet to solidify power. 
 
 Zhou Yongkang, China's former domestic security chief and one of the nation's top nine leaders until he retired in 2012, will be investigated for "serious disciplinary...
– Chinese President Xi Jinping has made it his mission to eliminate corruption within the ruling Communist Party, and his latest target is the highest-ranking thus far. Zhou Yongkang is the former head of national security and an ex-member of the Politburo Standing Committee, which heads the party; he was among "the country's most feared leaders" before he retired in 2012, the AP notes. But he hasn't appeared publicly in months, the BBC reports, and now, state media say he's facing an investigation over a possible "serious disciplinary violation." Though unspecific, that language tends to refer to corruption, the Wall Street Journal notes. Zhou, a onetime ally of disgraced politician Bo Xilai, is the highest-ranking official to face investigation since the 1980s, when Mao Zedong's wife was among those targeted in an investigation, the BBC reports. Until now, such inquiries had avoided Politburo members, the Journal notes. Xi could face anger within the party over the probe, the paper adds, but it also points to the solidifying of his power, the BBC notes.