A prominent Taiwanese human rights advocate has been detained in China and is under investigation on suspicion of harming national security, the Chinese government has said.
It is the first official comment about Lee Ming-che since he went missing in southern China earlier in March.
Mr Lee is in good health, officials say. No details have been released about where he is being held.
He specialises in promoting human rights and democracy on social media.
The BBC's John Sudworth in Beijing says that his detention fits a pattern of increasing intolerance in China of human rights and civil society activists, including those based overseas.
Earlier this week China banned a visiting professor - a Chinese national but with permanent Australian residency - from leaving the country, apparently because of his human rights research.
Mr Lee - a regular visitor to China - went missing after entering the semi-autonomous Chinese territory of Macau on 19 March. He failed to attend a meeting later that day with a friend across the border in mainland China.
"Regarding Lee Ming-che's case, because he is suspected of pursing activities harmful to national security, the investigation into him is being handled in line with legal procedures," Taiwan Affairs Office spokesman Ma Xiaoguang said.
Friends and supporters of Mr Lee say the authorities may have been alerted after he used the social media platform WeChat as a forum to debate China-Taiwan relations.
Taiwan's governing Democratic Progressive Party (DPP) said over the weekend that China's failure to release details about Mr Lee was causing "anxiety and panic" to his family. It demanded better protection for Taiwanese people in China.
Mr Ma insisted however that said Taiwanese people coming to China for "normal" activities did not have anything to worry about.
"The mainland has rule of law," he said. "On this point, Taiwan compatriots can rest at ease."
Relations between the two countries have deteriorated over the last year, mainly because China is resentful of the DPP's traditional support for Taiwanese independence.
It is also extremely sensitive to criticism and has cracked down on domestic critics.