Sun Zhengcai had been party chief of the southwestern megalopolis of Chongqing, until an abrupt announcement on Saturday morning that he no longer had the position and had been replaced by a rising political star close to President Xi.
The announcement, carried by state news agency Xinhua, did not say Sun either had a new position or use wording to suggest he was waiting for a further appointment.
A source who has been briefed on the matter said Sun is suspected of "serious discipline violations", a term that can encompass everything from taking bribes to not toeing the party line. The source added that it was a "conversation investigation", meaning it's not yet at the stage of a formal probe.
A second source with ties to the leadership told Reuters that Sun is undergoing investigation for suspected "violation of political discipline". The source declined to elaborate.
"But he is still a comrade. He is still a Politburo member," the source said, referring to the party's 25-member decision-making body.
Officials are stripped of their title "comrade" once a formal legal case is filed against them and they are expelled from the party. Officials are sometimes put under investigation but not formally charged. However once a party announcement about a probe is publicly announced they are almost always punished.
The Communist Party's anti-corruption watchdog, the Central Commission for Discipline Inspection, did not respond to a request for comment on Sunday. Calls to the Chongqing government information office also went unanswered.
It was not possible to reach Sun directly for comment, and unclear if he has been allowed to retain a lawyer. It is not clear if any limits have been put on Sun's movements.
New Party Boss
Hong Kong and Taiwan media as well as the Wall Street Journal have also reported Sun is being investigated.
Sun did not appear on Chongqing television's Saturday evening broadcast when new party boss, Chen Miner, was introduced to city officials by Zhao Leji, head of the powerful organization department which oversees personnel decisions.
Zhao made the announcement about the probe at the Chongqing meeting, the first source added.
When Beijing's new party boss Cai Qi was unveiled in May, Zhao not only introduced Cai but Cai's predecessor Guo Jinlong was also present at the meeting and gave a speech, according to the Beijing city government's account of the event.
A third source with ties to the leadership emphasised how unusual it was that Sun was not present or mentioned at all in the meeting where Chen was presented as new Chongqing leader.
The source said the Chongqing meeting was told that Sun had committed "political mistakes".
The three sources, who all spoke on condition of anonymity as they were not authorised to speak to foreign media, said Sun was currently in Beijing.
Chongqing is perhaps best known for its association with its disgraced former party boss Bo Xilai, once himself a contender for top leadership before being jailed for life in 2013 in a dramatic corruption scandal.
Sun had been seen as a potential candidate for elevation at the autumn congress and as a possible future premier, but his star has waned.
Sources with ties to the leadership and foreign diplomats say Sun has been out of favor after the party's anti-corruption watchdog in February criticized Chongqing authorities for not doing enough to root out Bo's influence.
At that meeting, Sun said he accepted the watchdog's assessment "without question", according to a party statement at the time.
Chongqing is one of China's most important cities. Global electronics brands including Hewlett-Packard, Foxconn, Acer and Asus all have operations in Chongqing, lured by tax breaks, cheap labor and land, plus a developed supply chain and logistics. The region makes a third of the world's laptops.
Diplomatic sources say Sun had been scheduled to go on a trip to Europe recently but it had been cancelled. It is not immediately clear which countries he was due to have visited.
However, another candidate for elevation, Guangdong province party boss Hu Chunhua, visited Britain, Ireland and Israel last month. Such trips are often used by senior officials to raise their international profiles ahead of promotions.
Sun's last public appearance as mentioned by state media was on July 12, when he attended a Chongqing party meeting about deepening reforms.