The Hong Kong publisher whose disappearance has caused a major rift between Hong Kong and Beijing has written to a colleague to say that he is in China, where he is "cooperating with the authorities with an investigation."

Lee Bo, whose publishing company specializes in books critical of Beijing’s Communist Party leaders, vanished  Wednesday in what lawmakers and experts say looks increasingly like an illegal abduction by Chinese police. He is the fifth member of the Mighty Current publishing house to vanish, amid reports that it was planning a gossipy book about the love life of Chinese President Xi Jinping.

In a fax to a colleague at Causeway Bay Books, a subsidiary of Mighty Current, Lee claimed to have traveled to China "by my own means, to cooperate with an investigation carried out by relevant department," calling his situation "good" and "normal." But the handwritten fax, published by Taiwan's Central News Agency, raised as many questions as it answered, because Hong Kong police have said that they have no record of Lee passing through immigration, and his wife has said that he was not carrying any travel documents with him when he vanished.

In China, "assisting the authorities with an investigation" frequently equates to detention, suspicion of criminal activity and, sometimes, even torture. With the case garnering considerable international attention, the suspicion remains that Chinese police abducted Lee and that Chinese authorities pressured him into sending the fax in an attempt to calm the situation. On Monday, his wife, Sophie Choi, went to Hong Kong police to withdraw her earlier complaint, according to Hong Kong's Ming Pao newspaper.

"I think it's a charade performed under duress," said Claudia Mo, a lawmaker with the pro-democracy Civic Party. "He has obviously been smuggled out, but his wife has got the message that keeping a low profile would help his release, because the Chinese would save face."

Lee’s disappearance is seen as an assault on Hong Kong’s cherished principles of freedom of expression and autonomy from Beijing, as well as a sign that China is becoming increasingly bold in its efforts to track down and abduct dissidents and opponents outside its borders.

On Monday, Hong Kong’s chief executive, Leung Chun-ying, said it was “not acceptable” for Chinese police to operate independently in Hong Kong, but Leung, a Beijing loyalist, said there was “no indication” that this was what had happened.

Pro-democracy lawmakers said, however, that it appeared likely that Chinese police had kidnapped Lee.

If confirmed, the lawmakers said, Lee’s abduction would be a serious violation of the "one country, two systems" principle and the Basic Law framework that have defined Beijing’s relations with Hong Kong since the 1997 handover from British rule.

“This is a serious concern to all Hong Kong people,” Civic Party lawmaker Dennis Kwok said in a telephone interview. “This sort of thing is not supposed to happen in Hong Kong."

Anson Chan, who was the territory’s chief bureaucrat immediately after the 1997 transfer, said Hong Kongers were feeling increasingly vulnerable amid a “steady erosion” of the rights that were guaranteed at the time of the handover.

“If mainland authorities are allowed to enforce laws in Hong Kong, it spells the death knell for ‘one country, two systems,'” she said in a telephone interview.

Lee, a major shareholder in Mighty Current and Causeway Bay Books, disappeared Wednesday evening from Hong Kong, before reportedly calling his wife from a number in Shenzhen, southern China, saying he was assisting in an investigation.

In October, another major shareholder, Gui Minhai, disappeared from his beachfront apartment in Thailand, and three employees vanished around the same time while traveling in southern China.

On Monday, Leung called reporters to his office to “state solemnly” that, under the Basic Law, only Hong Kong’s law enforcement agencies have the authority to enforce laws in the territory.

“The freedom of the press and freedom of publication and freedom of expression are protected by laws in Hong Kong,” he said, according to an official transcript of his remarks, adding that he was paying close attention to the case.

Other officials said local police were pursuing Lee’s disappearance as a “missing persons” case and had contacted authorities on the mainland. But Chan said Leung should take the matter up at the highest level in Beijing.

“The Hong Kong government needs to get to the bottom of this very quickly, give a full and proper account to the people of Hong Kong, and put people’s minds at ease,” she said.

In a viral video, activist Agnes Chow issued an "urgent cry from Hong Kong."

"In the past we were safe because we lived in  Hong Kong and not in mainland China. However, the circumstances have changed with the abduction," said Chow, a former spokeswoman for the student organization Scholarism, which played a leading role in the 2014 pro-democracy protests in Hong Kong known as the Umbrella Movement. By Monday evening, the video had been viewed more than  850,000 times on Facebook and YouTube. "We feel that Hong Kong is not Hong Kong anymore, it is named as Hong Kong only."

Democratic Party lawmaker Albert Ho said at a news conference on Sunday that the city was “shocked and appalled” by Bo’s disappearance, according to the Associated Press.

Ho said one possible explanation for Lee’s disappearance was that the Mighty Current publishing company was being pressured to scrap plans for a book rumored to be about an old “girlfriend or mistress” of Xi.

Willy Wo-lap Lam, an adjunct professor at the Chinese University of Hong Kong, said the book was to have been titled “The Lovers of Xi Jinping” and would have covered the period when Xi held various official posts in Fujian province between 1985 and 2002, including after his marriage to well-known People’s Liberation Army singer Peng Liyuan in 1987.

Mighty Current is one of a handful of publishers in Hong Kong specializing in books highly critical of the Communist Party leadership, often based on extremely flimsy and unspecified sourcing.

Many of the books focus on political intrigue and infighting in the corridors of power, as well as gossip about the exploits of leaders’ families. The books, banned in mainland China, are bought both by Hong Kong residents and tourists from the mainland.

Lam said Chinese authorities had made an unpublicized decision in early 2015 to "eradicate or silence" those four or five publishing houses.

In Beijing, Foreign Ministry spokeswoman Hua Chunying said at a routine news conference that she had not heard about the case and had “no information.”

But the nationalist Global Times newspaper had heard the reports.

In an editorial, it said the case had been “sensationalized” in Hong Kong, arguing that there was no point in “political speculation” while the investigation was continuing. But it simultaneously launched an attack on the publishers.

Many of the books published by the group contained “maliciously fabricated content and seriously violated the right to reputation,” causing political rumors to spread back to the mainland, it wrote.

“Although Causeway Bay Books is located in Hong Kong, it actually makes a living causing a disturbance in mainland society, and damaged the harmony and stability of mainland society,” it wrote. “In the era of the Internet, the negative influence it created is not only limited in Hong Kong, but it is also infiltrating the mainland, and became a practical problem for the country.”

William Nee, a China researcher with Amnesty International in Hong Kong, said that although all the facts had not come to light, there was “good reason to believe” that mainland authorities were behind Lee’s abduction.

“The detentions of bloggers, democracy activists and independent journalists are unfortunately almost a daily occurrence in China, but thus far this sort of naked attack on freedom of expression has been unheard of in Hong Kong," he said. "This shocking case should also be seen in the context of a creeping internationalization of China's crackdown on dissidents and freedom of expression.”

Nee cited China’s collusion with Thai authorities to repatriate a democracy activist and a cartoonist in November, though the U.N. High Commissioner for Refugees had recognized them as refugees, and the way it cooperated with Burmese authorities in October to secure the return of the 16-year-old son of a human rights lawyer and activist who had fled from China.

Further internationalizing the issue is the fact that Gui is a naturalized Swedish citizen, while one report in the Hong Kong media quoted Lee's daughter as saying that he might also hold a British passport. A spokesperson for the British Embassy in Beijing told the Guardian newspaper that British officials are "urgently investigating" those reports and are "deeply concerned" about the reports of the disappearances.

In Hong Kong, Lam said Lee’s disappearance has sparked huge concern among critics of the Chinese government, including pro-democracy politicians, journalists and other publishers.

“This could happen to anybody,” he said. “People are very shocked, and they are quite afraid.”

See more Harry Harrison cartoons in the South China Morning Post here.

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