The mysterious disappearance of a bookseller in Hong Kong has spooked residents, raising the possibility of Chinese security officers secretly entering the city and abducting individuals they are investigating.
More broadly, the case has exacerbated concerns about the "one country, two systems" framework which is supposed to ringfence Hong Kong's legal jurisdiction from the mainland after its return to Chinese rule in 1997.
Yesterday, Chief Executive Leung Chun Ying sought to quell such fears, stating categorically that it will be "unacceptable" for mainland agents to enforce the law in Hong Kong .
"The freedom of the press and freedom of publication and freedom of expression are protected by laws in Hong Kong," he said.
Under the city's mini-Constitution, the Basic Law, only agencies in Hong Kong have the legal authority to enforce laws in the city, he added. "No other law enforcement agencies - outside of Hong Kong, that is - have such authority."
Mr Leung noted widespread rumours that mainland officers are behind the sudden vanishing of Mr Lee Bo, a shareholder in publisher Mighty Current which puts out salacious and politically sensitive tomes about China's leaders.
When asked about the disappearance of Mr Lee, Chinese Foreign Ministry spokesman Hua Chunying said at a regular briefing yesterday: "I'm not aware of the situation; I have nothing to offer."
But an opinion piece in China's nationalistic Global Times on the same day left in no doubt official sentiments of such books.
The books sold by Causeway Bay Bookstore - run by Mr Lee and four associates who have also gone missing - are created out of "evil intent", and inflict great harm on reputations and are destabilising, it said. "These books spread to the mainland by various means, becoming a source of political rumours, and creating negative effects," said the article, signed by Shan Renping, a pen name for the newspaper's editor Hu Xijin.
The offending title of the book that purportedly led to Mr Lee's disappearance is Xi Jinping's Lovers, China specialist Willy Lam told The Straits Times, citing sources close to the publishing house.
Mr Lee's wife reported his disappearance last Friday. She told local media that she received a phone call from him last Wednesday night, when he disappeared.
She said he told her that he was "assisting in an investigation", but gave no details. Oddly, he spoke in Mandarin instead of the Cantonese they converse in, she said. The number he rang from was a Shenzhen number.
The Hong Kong authorities have told Mrs Lee that there are no records of her husband leaving the city. This raises the likelihood that he was smuggled out via "underground" channels, Democratic Party lawmaker Albert Ho, a lawyer and human rights activist, told The Straits Times.
In the past two days, small groups of protesters have rallied to demand answers to Mr Lee's disappearance.
Mr Leung said there is "no indication" that mainland legal agencies have spirited Mr Lee away, but added that the government is extremely concerned about the situation.
Acting Secretary for Security John Lee said the police have "actively investigated", including checking CCTV footage of places where the bookseller was last seen, and will be looking for other leads.
Mr Lee's four colleagues disappeared last October.
However, Mr Lee's case has sparked extra attention in Hong Kong because he disappeared from the city; the others - all mainlanders - vanished from Thailand and mainland China.
Causeway Bay Bookstore in the shopping mecca of the same name is among various outlets in Hong Kong popular with mainland tourists looking for titles banned in mainland China.
More importantly, it is the biggest of about four or five Hong Kong-based publishers of Chinese political gossip, accounting for about 30 per cent of the output.