China confirms Hong Kong bookseller investigation
Police in China have confirmed for the first time that they are investigating three Hong Kong booksellers.
In a letter to Hong Kong's police, they said Lui Por, Cheung Chi Ping and Lam Wing Kee were suspected of "illegal activities".
The three are among five men linked to a Hong Kong bookshop and publishing house to go missing in recent months.
Some people in Hong Kong believe they were detained by China because of a book about President Xi Jinping.
No further information was given on the location or status of the men, and Hong Kong police have not made the letter public.
Causeway Bay Books and the Mighty Current publishing house which owns it are known for producing and selling works critical of China, which are banned in the mainland.
A US-based Chinese writer has told the BBC he was the co-author of the book some believe to be at the centre of the case.
He said he had now published the book online to challenge China, and that he should be held responsible, not the booksellers.
'Letter from Lee Bo'
Hong Kong police said the letter they received said Mr Lui, Mr Cheung and Mr Lam were suspected of being involved in the case of another of the missing booksellers, Gui Minhai.
They had "criminal compulsory measures" put on them, said the letter, according to Hong Kong police.
Mr Gui, a Swedish citizen and the owner of the Mighty Current publishing house, disappeared from Thailand in October.
He recently appeared on Chinese state TV saying he had voluntarily handed himself over to the authorities over a historic drink-driving fatality.
Hong Kong police said they had also been given a handwritten letter from the fifth bookseller, British passport-holder Lee Bo, in which he turned down a request to meet Hong Kong police.
Mr Lee disappeared from Hong Kong in October - a previous letter from him said he had gone to China voluntarily.
Under the "one country, two systems" principle, Hong Kong is supposed to enjoy high degrees of autonomy from China.
The case has sparked international concern that China could be attempting to rein in freedom of expression in Hong Kong.
Mighty Current publishing house disappearances
1. Lui Bo, General Manager, goes missing in Shenzhen, 15 October
2. Cheung Jiping, business manager, 32, goes missing in Dongguan, 15 October
3. Gui Minhai, co-owner, 51, goes missing in Thailand, 17 October
4. Lam Wingkei, manager, 60, last seen in Hong Kong, 23 October
5. Lee Bo, shareholder, 65, goes missing in Hong Kong, 30 December