At least five men associated with Mighty Current, which publishes titles critical of China's leaders, and bookstore Causeway Bay Books have gone missing since October.
In a statement published by the Hong Kong government Thursday said that Lui Por, Cheung Chi Ping and Lam Wing Kee are "under investigation" and being held by police in Guangdong province.
This was the first time Chinese police revealed that the three men -- who went missing while visiting the mainland last year -- were in their custody.
The men are "suspected to be involved in a case relating to a person surnamed Gui," the statement said. Gui Minhai, co-owner of Causeway Bay Books, who went missing in Thailand, reappeared on Chinese television last month tearfully confessing to a 2003 hit-and-run incident
A Swedish passport holder, Gui said he voluntarily turned himself into Chinese authorities.
However, Thai police told CNN they have no record of him leaving the country
and a police official, speaking to CNN on condition of anonymity due to the sensitivity of the case, says Thai authorities are now working with Swedish officials to investigate Gui's possible kidnapping.
It's also unclear how Lui, Cheung and Lam could be connected to Gui's hit-and-run. The statement added that they were "involved in illegal activities on the mainland."
"Nearly four months after the men first vanished, we still don't know where they are detained, on what charges they are being investigated, and whether they are allowed to see their lawyers or families," said William Nee, China Researcher at Amnesty International.
"The Chinese authorities seem to think that if they can get detained people under their control to write implausible letters or call family members saying that they are pro-actively 'cooperating with investigations' they can do away with due process and human rights."
Almost complete picture
While Chinese police have confirmed that they are holding four of the five missing men, the status of another, Lee Bo, remains unclear.
Lee, a British passport holder, disappeared from Hong Kong in January
, sparking mass protests amid claims that he had been detained and spirited across the border by Chinese police, who are not permitted to act in the autonomous city.
Since his disappearance, several letters purportedly written by Lee have been published in Hong Kong newspapers, and his wife, Choi Ka-ping, claims to have met with him in a guesthouse on the mainland
"I went to the mainland to cooperate in an investigation voluntarily," the most recent letter said.
"Solving a problem takes some time, it involves many of my private and company's internal affairs."
Hong Kong police said they are continuing to investigate the incident and have requested the Guangdong Public Security Department assist in arranging a meeting with Lee.
In a regular press briefing Monday, U.S. State Department spokesman John Kirby said the disappearances of Lee and Gui "raise serious questions about China's commitment to Hong Kong's autonomy under the 'one country, two systems' framework as well as its respect for the protection of universal human rights and fundamental freedoms."
Xi Jinping's lovers
Hong Kong lawmaker Albert Ho told CNN that Mighty Current had been planning on publishing a book about the "love affairs" of China's President Xi Jinping during his time working "in the provinces."
At least two books purporting to be that title have appeared online this month, perhaps suggesting that the attention around the case has prompted some to want to cash in on the massive media exposure.
One book, originally sold on the Google Play Android store, is no longer available. Another charges around $12 to download from Google Books, though extensive excerpts are readable for free. Both are titled "Xi Jinping and his lovers."
The latter book is co-authored by Xi Nuo, who has previously written titles discussing the love life of disgraced Chinese security tzar Zhou Yongkang
and a biography of Gu Kailai, the wife of former high-ranking Communist official Bo Xilai, who was convicted of murdering British businessman Neil Heywood
CNN was unable to independently confirm the authorship of the two books. Uniepress, the publisher of Xi Nuo's title, did not respond to a request for comment.