Guangdong police confirmed on Thursday for the first time that three Hong Kong booksellers who had not been heard of since they went missing last October were being investigated in mainland China. They also told Hong Kong police that missing Causeway Bay Books owner Lee Po, who is also on the mainland, had rejected their request to meet him. READ MORE: Causeway Bay Books remains closed despite claim by Lee Po’s wife: four booksellers still missing, presumed detained After Gui Minhai, another bookseller missing since October, earlier said on state TV that he had turned himself in over a fatal accident he was involved in 12 years ago, the Guangdong Provincial Public Security Department said on Thursday night that his three colleagues were suspected to be involved in Gui’s case and were also “involved in illegal activities on the mainland”. This was the first time Guangdong police had confirmed that Lui Por, Cheung Chi-ping and Lam Wing-kee were in custody on the mainland. READ MORE: Missing Hong Kong bookseller Lam Wing-kee is a ‘super book lover’ who used to be boss of Causeway Bay Books “Criminal compulsory measures were imposed on them and they were under investigation,” Guangdong police wrote in a letter to their Hong Kong counterparts. They enclosed a letter from Lee stating that relevant authorities had informed him of a request by Hong Kong police to meet with him on the mainland. He wrote that he “did not need to meet with them at the moment” and would contact them if he wished to do so. Lee’s wife confirmed his handwriting when she was shown the letter. READ MORE: Vanishing freedoms? Disappearance of bookseller Lee Bo raises questions about jurisdiction and rights in Hong Kong Hong Kong police last night said they had written again to the Guangdong side requesting help in following up the case of Lui, Cheung and Lam, and asking them to pass on a message to Lee that police still wanted to meet with him as soon as possible. The latest development is unlikely to curb speculation in Hong Kong that the booksellers were kidnapped and taken to the mainland by security agents from across the border acting beyond their jurisdiction. It is widely believed they got into trouble for selling books critical of the Chinese Communist Party. The piecemeal release of information and questionable explanations by mainland authorities have added to fears that Hong Kong’s autonomy has been undermined. Of the five booksellers, Gui, a Swedish national, had been missing for the longest. He appeared on CCTV earlier, claiming he had been fleeing from a suspended two-year jail term since causing the death of a 23-year-old university student while drunk-driving in Ningbo, Zhejiang province in 2004. The CCTV report also stated Gui had been involved in other criminal activities, and the “related persons” were also being investigated. Gui reportedly disappeared while on holiday in Thailand in October. Lee, a major shareholder in Causeway Bay Books, went missing in Chai Wan on December 30.