Jia Jia, a popular Chinese columnist who went missing last week before boarding a flight to Hong Kong, has been detained by Beijing police, his lawyer said Monday. Yan Xin reportedly said police told him that Jia, a frequent commentator on political and social affairs, was taken into custody at the airport.

According to BBC, Yan wrote on his WeChat account that police said Jia was "suspected to be involved in a certain case" but the details of his arrest were not given out. Before his scheduled departure, Jia had told his friends he believed something could happen to him after he had warned former colleagues about re-publishing an open letter calling for President Xi Jinping to resign.

“There were police officers from the airport branch assisting in the case, so they could confirm Jia was taken away,” Yan reportedly said in comments to the South China Morning Post.

Jia is said to be linked to the letter — published on a state-linked site Wujie News, or Watching, earlier this month — calling for Xi’s resignation. The letter was later taken down by the website, and Jia denied he had anything to do with the letter itself. Jia wrote a regular column for Tencent Online.

The news of Jia’s disappearance last week came amid the mystery over five Hong Kong booksellers, who sold books critical of the Chinese government and banned on the mainland. The booksellers were reported missing, only to resurface in Chinese custody.