A Taiwan-based book publisher has been placed under investigation in China on suspicion of “endangering state security,” Beijing said Tuesday amid mounting concern over his disappearance.

Li Yanhe, better known by his pen-name Fu Cha, was reportedly detained by police in Shanghai in March, shortly after he arrived to visit his family and deal with residency-related issues, according to Taiwan’s Central News Agency (CNA).

Weeks after his reported detention, a spokesperson of China’s Taiwan Affairs Office confirmed Wednesday Li is being investigated by state security authorities for his “suspected engagement in activities that endangered state security.”

Under leader Xi Jinping, China has stepped up efforts to crackdown on dissent – both within and outside its borders.

Li’s detention comes at a tense moment in cross-strait relations, and several Taiwan citizens have been detained in China on state security grounds in recent years.

China’s ruling Communist Party claims Taiwan as its own territory, despite having never controlled it, and has refused to rule out the use of force to “unify” the island with mainland China.

On Tuesday, Chinese authorities announced the formal arrest of Yang Chih-yuan, a pro-independence political activist from Taiwan, on suspicion of “secession”, more than eight months after he was detained in eastern Zhejiang province, prompting Taiwanese authorities to urge citizens to evaluate the risks before traveling to mainland China.

Taiwan’s Minister of Mainland Affairs Council Chiu Tai-san said Wednesday the arrest of Yang and the detention of Li showed that “China is exercising its long-arm jurisdiction to Taiwan not only to threaten but also to suppress Taiwan.”

Born in China’s northern Liaoning province in 1971, Li moved to Shanghai in his 20s to study literature and later worked in the publishing industry.

He relocated to Taiwan in 2009 after marrying a Taiwanese woman. In Taiwan’s capital, Taipei, he founded Gusa Publishing, which published books critical of the Chinese Communist Party or touching on China’s political taboos, such as the Tiananmen Square massacre, according to the CNA.

Wang Dan, a student leader during the 1989 Tiananmen Square protests, said Li’s Gusa Publishing had published a collection of his essays.

According to Wang, Li had obtained Taiwan citizenship and went back to Shanghai in March seeking to renounce his Chinese citizenship as required by Taiwan’s immigration law.

CNN has reached out to Taiwan’s Mainland Affairs Office to inquire about Li’s citizenship status.

Last week, an official with Taiwan’s Mainland Affairs Council said Li was “safe,” but declined to provide further details citing the wishes of his family.

Gusa Publishing said in a statement Monday on Facebook that it will stop commenting on the case out of respect for Li’s family but thanked the public for its support for Li. It declined to comment when contacted by CNN on Wednesday.

Not an extremist

Wang Chia-hsuan, a former editor at Gusa who worked with Li for eight years, said he and other friends of Li had always been concerned about his personal safety whenever he went back to China.

Wang Chia-hsuan said Li was dedicated to his work and longed for a different approach to understanding Chinese history.

“He is not an extremist. All the work he published is of academic nature and is already out in the public domain,” Wang said.

A group of 40 authors who have been published by Gusa and other supporters, including Wang Dan and Wang Chia-hsuan, have issued a joint statement calling for Li’s release.

“In Taiwan, freedom of speech and publication, and academic freedom are like the air we breathe. They are part of daily life for every reader, every author, every translator and every editor,” said the statement.

“Under Fu Cha’s leadership as editor-in-chief, Gusa’s books have been very diverse and deeply popular with Chinese-language readers around the world. We believe Fu Cha has not committed any crime in enjoying the freedoms of speech and publishing,” it said.

Li’s detention has drawn comparisons with the case of five Hong Kong booksellers.

The five, connected to a bookstore that sold critical and sometimes gossipy titles about China’s elite, went missing in late 2015 and eventually turned up in police custody in China.

One of them, Lam Wing-kee, has said he was kidnapped by Chinese “special forces” after crossing the border into mainland China from Hong Kong. He jumped bail in 2016 and fled Hong Kong for Taipei, where he reopened his bookstore.

Another, Gui Minhai, who went missing during a holiday in Thailand, was sentenced in 2020 to 10 years in prison for illegally providing intelligence overseas.

Hong Kong used to be a hub for publishing politically sensitive books that would be banned in mainland China.

But as Beijing tightens its grip on the city, especially after the enactment of a sweeping national security law, that role has been taken over by Taiwan – a vibrant self-governing democracy.