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People take part of the rally on January 10, 2016 in Hong Kong, Hong Kong. The disappearance of five Hong Kong booksellers, including UK passport holder Lee Bo, has sent shivers through Hong Kong as anxiety grows that Chinese control over the city is tightening.
The next day, his wife received a call from Lee, saying that he was "assisting with an investigation" in the Chinese border city of Shenzen.
His wife, however, discovered important travel documents that Lee would need to cross the border still at home and promptly reported him missing to Hong Kong authorities.
Lee has not been heard from since.
Lee's status as a known critic of Beijing and the Communist Party lead many in Hong Kong and abroad to speculate that he's being forcibly detained by the Chinese.
"If you put two and two together, he could have only been abducted [by Chinese police]," Willy Lam, a legal scholar at the Chinese University of Hong Kong, told TIME.
"In recent interviews, Lee said he hadn't been to the mainland for two or three years [because of fears for] his safety. I doubt he changed his mind," Lam continued.
Lee isn't the first person in his company to go missing under mysterious circumstances either. Gui Minhai, Lui Por, Cheung Ji-ping, and Lam Wing-kei, all of Causeway Books, a company that publishes books critical of China's communist party in Hong Kong, all disappeared in the last quarter of 2015.
The disappearances, and the frustrating investigations that resulted, have heightened tensions in Hong Kong and cast doubt on the efficacy of the "one country, two systems" policy under which Hong Kong and mainland China have coexisted since the British ceded control of Hong Kong to China in 1997.
Some of the books for sale from Causeway Books, many of which feature Chinese President Xi Jinping on the cover.
Hong Kong's constitution protects Causeway's publications under the banner of "free speech," but mainland China has no such right guaranteed to their citizens and a bad reputation for silencing dissenters.
As a result of these tensions, thousands of protesters took to the streets of Hong Kong on January 10 bearing pictures of the missing booksellers. Their message was clear: They would not live in fear of oppression or be denied their constitutional rights.
Lee's disappearance, if perpetrated by mainland Chinese authorities, shows a new level of disregard for Hong Kong's sovereignty. Gui, Lui, Cheung, and Lam, all disappeared while traveling around mainland China and Thailand, while Lee appears to be the first to go missing directly from Hong Kong.
By law, Chinese authorities must report detaining any Hong Kong resident within 14 days, a deadline fast approaching.
On Tuesday morning Hong Kong's Chief Executive Leung Chun-ying seemed to caution against optimism.
"Sometimes it takes longer, and sometimes the otherwise, for mainland authorities to respond on cases like these... this case was reported to the Hong Kong police on January 1, and it has been 11 days since then, but there were cases in the past which took longer for mainland authorities to respond to," he said, according to the South China Morning Post via Forbes.
"I have relayed Hong Kong people's concern to the relevant departments. We are following up and seeking help from different levels and sources," Leung continued.
Causeway Books is located in Hong Kong's Causeway Bay, one of the most lavish and Western-like retail districts in the city. In cramped second-story storefronts, tourists from the Chinese mainland pour in to browse and purchase books on prominent figures of China's Communist Party.
The books, which are illegal in the mainland, often address the political and personal scandals of Communist Party members, who control much of the media produced on the mainland and likely see the protected speech and independent reporting of the Hong Kong booksellers as a threat to their regime.
For now, Causeway Books is closed, with the remaining staff fearing for their safety.