Communist China’s Taiwan Affairs Office confirmed on Wednesday that a Taiwanese man who disappeared last month is in Chinese custody and is under investigation for “activities that endanger state security.”

Details of the allegations against Lee Meng-chu were not provided. Lee’s friends and family reported losing contact with him after he arrived in Hong Kong on August 18 and made his way to Shenzhen, the nearest Chinese city. Shenzhen police officials claimed they had no record of him entering the city, even though friends living in Shenzhen said they had dinner with him.

Suspicions arose that Lee was detained for taking photos of the Chinese paramilitary troops and equipment massing in Shenzhen for a potential invasion of Hong Kong. One of his photos was published by Taiwanese media.

“We have no idea whether he has been tortured and forced to admit to any crime,” said Chen Ya-lin, head of the fishing town in Taiwan where Lee lives and works for a volunteer group.

“Though he has been enthusiastic about public affairs, he has never engaged in politics, and what China did to him is not only unreasonable but also shocking,” Chen said.

The office of Taiwanese President Tsai Ing-wen said on Wednesday it was “deeply concerned about the detention of our national Lee Meng-chu” and has instructed Taiwan’s Mainland Affairs Council and other agencies to “negotiate in full force with the Chinese side about the issue.”

The Mainland Affairs Council duly asked Chinese authorities to “provide detailed information about where he was detained and why he was restricted of his personal freedom in line with the agreement on cross-strait joint efforts to fight crimes.”

The council also asked for arrangements to be made so Lee’s relatives and legal representatives could visit him.

Radio Free Asia on Wednesday quoted speculation in Taiwan that Lee might have been grabbed by China to express displeasure with Taiwanese support for the Hong Kong protest movement, or because he might have contacted someone in the Chinese human rights activist community. Taiwanese officials mentioned that Chinese border police are intrusively searching the phones and computers of travelers from Hong Kong for protest-related material.