Three missing Hong Kong booksellers are under currently investigation by mainland authorities, police confirmed on Thursday.

Lee Bo (left) and his four colleagues at the Causeway Bay Bookstore have gone missing since October.

Lui Por, Cheung Chi Ping and Lam Wing Kee are suspected of being involved in a case related to a fourth bookseller in custody, Swedish national Gui Minhai, according to a letter Hong Kong police received from the Interpol Guangdong Liaison Office of Guangdong Provincial Public Security Department on Thursday afternoon. It is the first confirmation of the trio’s whereabouts.

Gui is co-owner of Causeway Bay Books. He disappeared in Thailand last October but reemerged on Chinese state television last month to “confess” to a crime he allegedly committed over a decade ago, saying that he turned himself in to Chinese authorities. Gui left China after reportedly receiving a suspended sentence for killing a 20-year-old girl whilst drink-driving in 2003.

Gui’s daughter has told local media that she believed her father was kidnapped and taken to China.

A total of five people from the Hong Kong Mighty Current publishing house have gone missing since October. The company specialises in politically sensitive titles banned in mainland China.

New letter from Lee Bo

Hong Kong police also received a handwritten letter from Gui’s colleague, Lee Bo. Lee declined a request by Hong Kong police to meet with him. He stated that he “did not need to meet with police at the moment” but would contact them if necessary. Police confirmed the handwriting in the letter with Lee’s wife.

Lee, a British citizen, disappeared from Hong Kong just before the new year and is now confirmed to be in Guangdong province. He also said he voluntarily returned to China for an investigation.

See also: More on the missing publishers case.

Tom is the editor-in-chief and founder of Hong Kong Free Press. He has a BA in Communications and New Media from Leeds University and an MA in Journalism from the University of Hong Kong. He has contributed to the BBC, Euronews, Al-Jazeera and others.