One of the five missing Hong Kong booksellers associated with Causeway Bay Books is a British national, a spokesperson for the British embassy in Beijing said, according to Reuters. The five people, including British citizen Lee Bo, are suspected to have been abducted to end a “smear campaign” against Chinese president Xi Jinping, the Guardian reported, citing a source.

According to the BBC, Lee had alerted authorities when four of his colleagues from Causeway and another related publishing house, Mighty Current, disappeared. The men went missing since mid-October from southern China, Thailand and Hong Kong. Causeway Bay Books is known to have published material critical of China’s Communist Party. 

Lee's disappearance since last Wednesday has sparked concerns in Hong Kong that Chinese law enforcement agencies may have taken him to mainland. The report by the Guardian, published Tuesday, comes after Taiwan’s Central News Agency released a handwritten letter Monday, allegedly penned by Lee. In the letter, which was faxed to Lee’s colleague, he reportedly said that he traveled to mainland China on his own to help with “an investigation which may take a while.”

The letter, allegedly written by Lee, did not explain how he reached mainland without his travel documents and escaped immigration officials. “I am now very good and everything is normal,” the letter read. Reports also said Tuesday that, after receiving a copy of the letter, Lee’s wife tried to withdraw the missing person's report.

"We are deeply concerned by reports about the disappearance and detention of individuals associated with the Causeway Bay Books bookstore in Hong Kong," the spokesperson said, in a statement from the British embassy in Beijing, according to Reuters, adding: "We encourage the Hong Kong SAR government to honor its commitment to protecting the freedom of the press, and we hope the Chinese authorities will continue to make every effort to ensure that the environment in which the media and publishers operate in the Hong Kong SAR supports full and frank reporting."

Lee's colleague, Gui Minhai, a Swedish national, disappeared in October while he was on a holiday in Thailand. The Swedish foreign ministry also said, according to CNN, that it was investigating reports that one of its nationals was detained in Thailand or China.

Politicians and activists, meanwhile, said the disappearances were conducted by Chinese authorities, indicating that Beijing was trying to suppress the civil rights of Hong Kong, which belongs to China but maintains its own legal, political and economic systems. The Guardian report also cited an interview with Lee, a month before his disappearance, where he suspected that Gui's disappearance might have been related to a book that Gui was planning to publish. The Guardian cited a source, who knew about the contents of the book, to say that the book was to be called "Xi and His Six Women."

"Lee Bo’s case is a game changer. It shows that ‘one country, two systems’ has completely collapsed," prominent Hong Kong publisher Bao Pu said, talking about the political system that was implemented in Hong Kong after a handover in 1997, guaranteeing the freedom of the territory’s residents.

China’s state-run Global Times said Tuesday, according to the Wall Street Journal, that Lee’s bookstore was selling political works with fabricated content. "Although the bookstore operates in Hong Kong, it has caused damage across the border in the rest of the country, which Lee Bo should have known," and added that Lee probably opted for a "low profile."

Albert Ho, a pro-democracy lawmaker, told CNN, "It's a forced disappearance. All those who have disappeared are related to the Causeway Bay bookshop and this bookshop was famous, not only for the sale, but also for the publication and circulation of a series of sensitive books."

He added: "The government has a duty to assure Hong Kong people that they are protected under one country, two systems by our law. Not only are mainland laws inapplicable in Hong Kong, no mainland officials, including law enforcement agencies, can take the law into their own hands in Hong Kong."