TAIPEI.- A Taiwan-based publisher who disappeared while in China has been detained for suspected violations of security laws, Chinese authorities confirmed Wednesday, fanning concerns in Taiwan that Beijing is sending a warning to the island’s vibrant publishing sector.

The publisher, Li Yanhe, widely known by his pen name, Fu Cha, is a Chinese citizen who has been living in Taiwan since 2009. His company, Gusa Publishing, is well known in Taiwan for books that cast a critical eye on China’s ruling Communist Party. Li had returned to China early last month to visit relatives but fell out of contact shortly after, according to his colleagues and friends.

Li’s detention is “a strong blow and will have a chilling effect,” Bei Ling, a writer from China living in Taiwan, said Wednesday. “Publishing houses, publishers and freedom of the press are the basic indicators of an open society all over the world. I don’t think he should be condemned in this way just because he published books that are unacceptable to China.”

Li’s detention could become a new test case in already tense ties between Taiwan and China. Beijing asserts that Taiwan, a self-governing democracy, is a part of Chinese territory that must accept unification. But many in Taiwan reject Beijing’s claim and have been repelled by the authoritarian rule of China’s leader, Xi Jinping, including his crackdown in Hong Kong, which has chilled publishing and stifled many liberties such as the right to protest.

Li was being investigated on suspicion of “engaging in activities endangering national security,” Zhu Fenglian, a spokeswoman for the Chinese government’s Taiwan Affairs Office, told reporters in Beijing. She did not give any details of the accusations against Li but said that China would protect his legitimate rights.

In China, national security crimes can mean anything from espionage to criticizing the Communist Party. People accused of such offenses can be detained for many months with no contact with family members or lawyers.

To Li’s supporters, the case carries echoes of the disappearance of five Hong Kong booksellers in 2015. Chinese investigators had secretly detained the men over their involvement with a publisher of books offering scathing, lurid descriptions of Xi and other Communist Party leaders. One of the booksellers, Gui Minhai, is serving a 10-year prison sentence on charges of providing intelligence abroad. The others were eventually released after making confessions that were broadcast by Chinese state media.

The investigation into Li was the second politically loaded case linked to Taiwan that China confirmed this week. Prosecutors said Tuesday that a Taiwanese man, Yang Chih-yuan, was formally arrested and accused of “separatist activities.” Yang is the vice chairman of the Taiwanese National Party, a small party that promotes independence for Taiwan. Zhu, the spokesperson for China’s Taiwan Affairs Office, said that his case was “a further wake-up call for Taiwanese separatist forces.”

The publisher, Li, was born in northeast China in 1971. He established himself as a publisher in Shanghai, where he worked for the Shanghai Literature & Art Publishing House, before he moved to Taiwan.

He took pride in being a descendant of the Manchu, the ethnic group that ruled China as the Qing dynasty from 1644 to 1912. The Chinese name of his publishing house in Taiwan that Li founded in 2009 means “Eight Banners,” a reference to the administrative divisions of Manchu rule.

Gusa Publishing produces a wide range of books, including many translations. Its offerings are dominated by mainstream nonfiction such as “The China Record: An Assessment of the People’s Republic of China” by Fei-Ling Wang, a professor at the Georgia Institute of Technology, and a translation of “The Long Game: China’s Grand Strategy to Displace American Order” by Rush Doshi.

Taiwan’s authorities may have a particularly difficult time gaining access to Li while he is in custody in the mainland, given the tensions between the two sides and that he remains a citizen of the People’s Republic of China.

“Fu Cha was born in mainland China, and it’s quite possible that he would not be allowed to leave mainland China because of this,” said Lam Wing-kee, one of the booksellers from Hong Kong who was detained in China in 2015. Lam has lived in Taiwan since 2019. He said: “Publishers in Taiwan should be careful, because you can’t ever change China’s mind.”

Li’s wife, who is Taiwanese, has declined to speak publicly about his case.

Taiwan’s Mainland Affairs Council, which deals with relations with China, said in a written statement that it was in close contact with members of Li’s family.

“In dealing with this case,” the council said, “the government will — with the precondition of respecting the family’s opinions — use all the methods and channels it can to provide the most appropriate care and assistance.”

This article originally appeared in The New York Times.