Hong Kong - A Chinese billionaire who went missing from Hong Kong and is believed to have been abducted by mainland security agents is reportedly under investigation in connection with China's 2015 stocks crash.Local media say financier Xiao Jianhua disappeared from his apartment in Hong Kong's harbourfront Four Seasons hotel on Friday and his whereabouts are still unknown.Three statements purportedly from Xiao - a Canadian citizen - denying he has been kidnapped have appeared on his company's WeChat account and on the front page of a Hong Kong newspaper. It is illegal for mainland agents to operate in semi-autonomous Hong Kong and the case has sparked new fears that its freedoms are under threat from Beijing.According to the South China Morning Post, Xiao, founder of Beijing-based Tomorrow Group, is in mainland China and "assisting investigations" into the stock market turmoil of 2015. The Shanghai stock index tumbled nearly 40% over a period of little more than two months after peaking in mid-June that year. Authorities helped inflate the bubble by encouraging investments. But when it burst, officials quickly sought to pin blame on market manipulators.It is unclear how Xiao is being linked to the crash, but mainland investigators have targeted several investment executives on suspicion of insider trading since the stocks rout. Last week a former star hedge-fund manager was sentenced to more than five years in prison for market manipulation.Anti-corruption driveThe investigation into Xiao is also linked to China's disgraced ex-spymaster Ma Jian, the SCMP said. The former deputy head of China's ministry of state security, Ma was expelled from the ruling Communist Party in December on suspicion of taking bribes and "abusing power".Ma's case is linked to other top officials who were thought to pose a threat to Chinese president Xi Jinping.There has been widespread speculation that Xiao's disappearance was part of Xi's ongoing anti-corruption drive, which some critics believe has been used to target his political opponents.The campaign was launched after Xi took power in 2012 and has brought down government officials and corporate executives.Xiao is said to have acted as a broker for the Chinese leadership, including for Xi's family - but there were also reports that he could be connected to rival political clans. Analyst Ma Ngok said the targeting of tycoons was "partly political struggle and partly because of the crackdown on corruption".He added that some other big Chinese businessmen based in Hong Kong would now be likely to leave. "They just don't have the confidence that if something happens in Hong Kong the government or the police is going to help out, I think this is the sentiment now," said Ma, professor at Chinese University of Hong Kong.Xiao's case has echoes of the disappearance in 2015 of five booksellers known for publishing salacious titles about Beijing's leadership. One of the men, Lee Bo, vanished from Hong Kong triggering international condemnation and local protests.All resurfaced on the mainland.