In 2015 five Hong Kong booksellers disappeared and later resurfaced in mainland China in the hands of Chinese authorities. Now, there are concerns that Chinese tycoon Xiao Jianhua, who has not been seen since last week, has met a similar fate.

The first murmur came from overseas.

Mingjing News, a New York-based site that specialises in Chinese politics, reported that an unnamed Chinese billionaire had been arrested in Hong Kong and taken to mainland China last Friday evening by Chinese police and state security officials.

The next day, Hong Kong police received a request to investigate the whereabouts of the missing businessman.

Local police began their investigation and found that the subject of their enquiry had indeed re-entered mainland China on Friday, the same day he was detained.

A day later, on 29 January, however, a family member asked the police to close the case, saying the missing person had been in touch and was "safe".

By Monday, all of Hong Kong was wondering, who was the billionaire? What was his crime? Where was he? And had Chinese agents been operating on Hong Kong soil with impunity?

US-based news site Bowen Press soon named the tycoon as Xiao Jianhua, one of China's richest people, with a $6bn (£4.8bn) fortune, according to Hurun Report, a ranking of wealthy Chinese people.

Mr Xiao, who is in his mid-40s, was known to have been a student leader at the prestigious Peking University during the 1989 Tiananmen Square protests.

His business ventures were mainly in the financial services sector.

In 2014, the China-born tycoon denied that he moved to Hong Kong to avoid a Chinese government corruption investigation.

He was said to have been last seen on Friday at the Four Seasons Hotel in Hong Kong, where he was understood to be living.

The mystery deepened when Mr Xiao's company, the Beijing-based Tomorrow Holdings, apparently issued a statement on its official account on messaging app Wechat.

"I, Xiao Jianhua, am overseas receiving medical treatment," said the announcement, according to Hong Kong media.

"Everything is fine. It's business as usual at Tomorrow," said the statement.

The message seems to contradict the Hong Kong police report that Mr Xiao had returned to China.

On Tuesday, the company issued yet another statement, reportedly saying Mr Xiao was a patriot and a Communist Party stalwart.