A lawyer for prominent Chinese columnist Jia Jia, who went missing last week, has confirmed he has been detained by Beijing police.
Yan Xin said police had told him that Mr Jia was taken away at the airport last week, before boarding a flight to Hong Kong.
Mr Jia is said to be linked to a letter calling for the resignation of Chinese President Xi Jinping, published on a state-linked site earlier this month.
The letter was quickly taken down.
Mr Yan posted on his WeChat account that police said Mr Jia was “suspected to be involved in a certain case” but did not give any details.
Friends have told media outlets that they believe Mr Jia disappeared because he warned an editor friend, Ouyang Hongliang, after the letter was published on Mr Ouyang’s news site Watching, also known as Wujie News.
Rights group Amnesty International’s researcher William Nee also told the AFP that Mr Jia’s disappearance was “most likely” related to his involvement in the letter. Amnesty has called on people to appeal for Mr Jia’s release.
Mr Jia has reportedly denied he had anything to do with the letter itself.
Meanwhile, Mr Ouyang, who is Watching’s chief executive officer, is also thought to be missing.
Several of his friends have told the BBC that they have been unable to contact him since last week, and the BBC’s own attempts to contact him on his mobile have also failed.
The case comes after the recent controversial disappearance of five Hong Kong booksellers, who later were confirmed to be detained on the mainland.
The letter, attributed to “loyal Communist Party supporters”, had criticised Mr Xi for various political, economic and diplomatic decisions, as well as for creating a “personality cult”.
An excerpt from the letter
“On the cultural and ideological front, you [President Xi] have emphasised ‘party-surnamed media’ [an official campaign to get all media to toe the party line], and have disregarded the citizen aspect of the media, stunning the whole nation; you supported… low-standard people to become our literary representatives, disappointing the workers in the larger literary world; you have condoned cultural units in directly singing your praises; your wife Peng Liyuan’s sister took over the producing duties of the CCTV Spring Gala, causing everyone’s beloved and anticipated programme to become your personal propaganda tool.
Your condoning of a personality cult, not allowing ‘rash opinions of central leadership’, creating a ‘one voice party’ method – those of us who have gone through the Cultural Revolution can’t help but feel secretly worried – our party, country and citizens cannot go through yet another 10-year catastrophe!”
It was first posted on overseas website Canyu and appeared to be republished on Watching on 5 March, before it was taken down. Canyu’s version is still online.
China has seen a recent spate of censorship incidents amid a crackdown on dissidents, journalists and lawyers.