HONG KONG—Mystery surrounds the whereabouts of a Chinese-born Canadian billionaire reportedly seized in Hong Kong by mainland Chinese police, in a case that could rekindle concerns about overreach by Chinese law enforcement in the semiautonomous city.

Xiao Jianhua is in mainland China, according to Hong Kong’s South China Morning Post on Wednesday, citing an anonymous source close to the tycoon, one of China’s richest individuals. Overseas Chinese news sites said earlier that Chinese police officers escorted Xiao from his suite at the luxury Four Seasons Hotel last Friday and took him away. Such news sites carry reports of political gossip and corruption scandals that can be difficult to verify in tightly controlled China.

The reports contradict a front-page ad on the same day in the Ming Pao newspaper purportedly in Xiao’s name, denying he was taken against his will. That would be an apparent breach of the city’s mini-constitution, which prohibits mainland Chinese law enforcement agencies from operating in Hong Kong.

“Let there be no misunderstanding!” the statement said. “It’s not true that I’ve been abducted and taken back to the mainland.”

The Canadian Consulate said it was aware of the reports and had contacted authorities “to gather additional information and provide assistance.”

Xiao is the founder of Beijing-based Tomorrow Group. The 46-year-old is worth nearly $6 billion, making him China’s 32nd wealthiest person, according to the Hurun Report , China’s version of the Forbes Rich List.

It’s unclear why Xiao was targeted, but his case has parallels with that of five Hong Kong booksellers, who disappeared in 2015 only to turn up under control of the mainland authorities, sparking fears that Beijing was eroding Hong Kong’s wide autonomy and rule of law. The five sold gossipy books about China’s communist leaders that were banned on the mainland but popular with Chinese visitors.

China’s Ministry of Public Security and Foreign Ministry did not respond to requests for comment on Wednesday, a public holiday in China.

The Hong Kong police force said Wednesday they have asked Chinese authorities for help in following up on the case.

In response to an inquiry about Xiao, the police force said initial investigations showed the “subject” crossed into the mainland at a border checkpoint Friday. They had launched the investigation after receiving a request for assistance from a family member on Saturday but a day later, the relative asked to withdraw the report after getting word that he was safe.

Xiao is reported to have built his fortune in part because of close connections with the families of Communist Party leaders. In 2014, reports said he fled to Hong Kong following rumours he was the target of a graft investigation — reports he denied at the time.

A wide-reaching anti-corruption crackdown led by Chinese President Xi Jinping has snared dozens of executives at state companies. Chinese state media said in 2013 that Xiao controlled nine publicly listed companies and had stakes in more than 30 financial institutions.

The New York Times reported in 2014 that a company Xiao co-founded paid $2.4 million to buy shares in an investment firm held by Xi’s sister and brother-in-law.

Rupert Hoogewerf, the Hurun Report’s publisher, said Xiao holds the reputation of a “maverick visionary” who makes major decisions on his own and has gone to lengths to obscure Tomorrow Group’s holdings, which include banks and securities companies.

“He’s one of the kings of capital of China,” Hoogewerf said. “There’s not many people moving money in the same way.”

According to the Ming Pao ad, Xiao was out of the country for medical treatment and would address the media once he finished his treatment. It followed similar statements the company posted this week on its Twitter-like Sina Weibo account, which have since been deleted.

The ad said Xiao is a Canadian citizen and a Hong Kong permanent resident and enjoys protections of both, and also has an unspecified diplomatic passport. Xiao was named ambassador-at-large for Antigua and Barbuda in 2015, according to the Antigua government’s website.

The ad cited Xiao as saying he was a patriotic Chinese businessman who “has always loved the party and the nation, and has never participated in anything that would hurt the country’s interests or the government’s image, let alone support any opposition forces or groups.”

It’s not unusual for officials, executives and other individuals detained by Chinese security to be pressured into releasing messages to their relatives or on their social media accounts claiming that all is well. Such messaging, coupled with censorship of reporting online by domestic media, has been seen as part of the Chinese authorities’ efforts to tamp down concerns about the safety of the individuals concerned.

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Jean-Francois Harvey, a Hong Kong-based lawyer, said he expected the case would further alarm mainland Chinese investors and businesspeople who have closely watched Beijing’s anti-corruption crackdown and the country’s economic slowdown.

In the wake of the booksellers’ incident, Xiao’s disappearance is likely to further degrade the perception that Hong Kong, unlike China, has an independent judiciary that operates outside of the political system, he said.

“People will be questioning whether Hong Kong is a safe haven,” Harvey said. “I think the answer to that question is very, very clear now, sadly.”

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