In three bookstores selling political books visited by Reuters, owners declined to be publicly interviewed, citing the fear of mainland anger. Hong Kong, a former British colony handed back to China in 1997, is constitutionally guaranteed freedoms and autonomy from Beijing for 50 years but the series of disappearances has led to suspicions that mainland law enforcement officers were ignoring the law.
Lee Bo, 65, a shareholder of Causeway Bay Books and a British passport holder, went missing from Hong Kong in late December, although his wife has withdrawn a missing persons report saying he travelled to China voluntarily to assist in an unspecified investigation.
Four other associates of the publisher have previously been unaccounted for, since late last year.
Police said they continue to investigate and China has yet to clarify the fate of the men and whether they are in custody.
Bao Pu, a prominent publisher in Hong Kong, was openly critical of the events and said he suspected mainland officials had breached the law. His father, Bao Tong, was the most senior Chinese official jailed over the 1989 Tiananmen Square protests in Beijing.
"Nobody is safe in Hong Kong now," said Bao, who published the secret memoirs of Zhao Ziyang, a former Communist Party general secretary who was purged after the 1989 crackdown.
As of Thursday, over 500 publishers, writers, booksellers and members of the public had signed an online petition pledging to: "Not fear the white terror and uphold the principle of publication freedom". White terror is a term used to describe periods of political persecution by authoritarian regimes.
Britain and the United States have expressed concern about the disappearances.
China's Foreign Minister Wang Yi skirted a direct question on whether the men were under Chinese detention at a recent press conference. The Guangdong and Shenzhen Public Security Bureaux, and the Hong Kong and Macau Affairs Office in Beijing, have not responded to repeated Reuters' requests for comment.
China's state-run Global Times tabloid, however, wrote in a recent editorial that the booksellers were exercising an "evil influence" in China through their political books. The newspaper went on to say that it was "reasonable" for law enforcement agencies to "circumvent the law when they seek cooperation from an individual for investigation."
Political gossip books and exposes on Chinese leaders have been a lucrative niche market for Hong Kong booksellers catering to Chinese visitors accustomed to pervasive censorship of sensitive literature back home.
But now, some stores have distanced themselves from such books.
At the PageOne bookshop chain, a young sales assistant told Reuters some of these books had been pulled recently.
"This is the company's decision," he said. "I'm not very clear about it. We only have history books now."
A spokesperson for PageOne said the firm wouldn't comment.
At the People's Book Cafe, posters of Mao Zedong - the late founder of modern China - were hanging above mainland Chinese tourists scouring the aisles to buy banned books on China's leaders, including Mao himself.
Paul Tang, the owner, told Reuters, that in the event the industry for banned books collapsed in Hong Kong, he expected it to "migrate to other nearby countries" like Taiwan or Japan given the huge sustained demand from Chinese visitors.
At two other small, independent bookshops visited by Reuters; Insiders Books and Best Reading Bookstores, staff refused to comment about the disappearances.
The Causeway Bay Books shop, which has been linked to all five missing men, remains locked, while at the firm's warehouse in an industrial building where Lee was last seen in late December, stacks of books wrapped in brown paper were piled up outside the door.
One pile of books was on Chinese President "Xi Jinping's ultimate battle with the old Communists" according to an invoice glued to the side.
Lee told Reuters in November that "the only possible reason" for the disappearances of his associates was because of a new book they were going to publish, that some local media said was an expose on Xi Jinping's love life. Lee, however, declined to give specifics.
"I really hope that they will be found," said Angela Gui, the daughter of Gui Minhai, one of the other missing publishers who was an associate of Lee and a shareholder in Mighty Current Media which ran the shuttered Causeway Bay Books shop.
"And that they will be returned from wherever they are, as soon as possible," she told Reuters by phone from Britain.