DISCRETION is not a trait often associated with the glitzy shopfronts of Hong Kong’s Causeway Bay, but the low-key entrance to a small bookshop belies the store’s recent notoriety. Tucked between a chemist and a clothes shop, Causeway Bay Books (pictured) has become the centre of a mystery involving alleged kidnappings by Chinese secret agents, and of a fierce debate in Hong Kong about the former British colony’s autonomy under Chinese rule.

The door to the shop, up two flights of stairs, is now locked. The trickle of people going up are journalists and passers-by curious for news about five men connected with the shop who have disappeared in recent months. Many Hong Kongers fear that agents from China’s mainland may have been involved, and that the men were targeted because of the shop’s gossipy books. Titles recently on sale include “The Collapse of [President] Xi Jinping in 2017” and “Xi Jinping and the Elders: War at the Top”.

When China took Hong Kong back from Britain in 1997, it agreed to give the territory a “high degree of autonomy”. Outspoken critics of the Communist Party remained free to air their views without fear of being “disappeared” by police, as commonly happens to their counterparts on the mainland. Hence huge public interest in this case. “The midnight knock on the door is not something we have had to worry about in Hong Kong,” wrote a columnist in the territory’s leading English-language newspaper, the South China Morning Post (recently bought by a mainland businessman). “But if we do now, that would be the end of our way of life.”

Worries about skulduggery by the mainland’s agents have been growing since October, when four of the men disappeared. One of them was Gui Minhai, the owner of Mighty Current, a publishing house which controls the shop. Mr Gui, a Swedish citizen of Chinese birth, went missing while staying in Thailand. Three members of the shop’s staff—Lam Wing-kei, Lui Bo and Cheung Jiping—disappeared during visits to China’s mainland.

Ominous clues

But it was the disappearance of Lee Bo, a shareholder in the shop, that aroused the biggest concern because it occurred in Hong Kong itself. Mr Lee, who is a British citizen, failed to return home after visiting his warehouse on December 30th. His wife told a television news channel that Mr Lee had later telephoned her to say that he was “assisting in investigations” relating to the other disappearances. She said he had sounded harried, and, unusually for him, had spoken in Mandarin—the official language of mainland China—rather than in Hong Kong’s Cantonese tongue. Mrs Lee said her husband had called from Shenzhen, a mainland city adjoining Hong Kong. Yet police in Hong Kong said they had found no record of him having crossed the border. Mr Lee did not take his mainland travel permit, his wife said.

The authorities in Beijing have done little to dampen speculation. Wang Yi, the foreign minister, told journalists that Mr Lee was “first and foremost a Chinese citizen”—implying that Mr Wang’s British counterpart, Philip Hammond, who happened to be visiting China this week, had no right to poke his nose in (Mr Hammond said British diplomats had “urgently inquired” about Mr Lee’s whereabouts). A newspaper published in Beijing, Global Times, accused the bookshop of selling works that “viciously attacked” the mainland’s politics and said Mr Lee was “well aware” of the “harm” these books were causing across the border. “Hong Kong cannot be a special base for hostile forces to carry out activities [aimed at] subverting the country’s political system,” it said.

Officials in Beijing have long worried about Hong Kong’s role as a safe haven for outspoken critics of the Communist Party, and those who try to expose its secretive inner workings. There is speculation that the mainland authorities may have finally lost patience having got wind of reported plans by Mighty Current to publish a book about President Xi’s private life.

Even the Hong Kong authorities—usually reluctant to hint at any disagreement with the mainland—are sounding a bit anxious. On January 4th the chief executive, Leung Chun-ying, told a hastily arranged press conference that any unauthorised encroachment by mainland agents would be an “unacceptable” breach of the Basic Law, the territory’s mini-constitution. But he said there was “no indication” of outside involvement in the case.

On January 5th a Taiwanese news agency published a faxed letter purportedly from Mr Lee saying everything was “normal” and that he had gone to the mainland “using his own method”. His wife said the letter appeared genuine, and that she had withdrawn her request for police help in finding her husband. Many commentators believe that the letter was written under duress, however, and that it hinted at the abnormality of his crossing into the mainland. Mr Leung said investigations would continue into Mr Lee’s case.

Mr Leung’s popularity has already badly suffered as a result of his endorsement of the mainland government’s refusal to grant full democracy in Hong Kong. He would risk even greater opprobrium if it became evident that he was turning a blind eye to mainland snatch-squads. Despite his efforts, he will find it hard to dispel the territory’s fears.