The fate of five booksellers who disappeared from Hong Kong last year became clearer after Chinese police said the three men still missing were under investigation on the mainland for alleged “illegal activities”.

Last month it emerged that two of the five from Causeway Bay Books, which sold works critical of Beijing’s top leaders, were already on the mainland, with Gui Minhai said by state media to be under investigation for unspecified crimes and Lee Bo claiming to be “assisting an investigation”.

Late on Thursday, Hong Kong police said they had received a letter from their counterparts in Guangdong province, which borders Hong Kong, stating that Lui Por, Cheung Chi Ping and Lam Wing Kee — the three still missing — were suspected of involvement in a case related “to a person surnamed Gui” and were “subject to “criminal compulsory measures”.