Mainland Chinese authorities have confirmed they're investigating three missing Hong Kong booksellers for unspecified criminal activity.

Hong Kong police said late Thursday they were told by police in neighboring Guangdong province about the men, who are linked to a publishing house specializing in politically sensitive titles banned on the mainland.

It's the latest development in a case that's gripped Hong Kong with rising concern that Beijing is clamping down on freedom of expression in the semiautonomous Chinese city.

The letter from Guangdong's public security department said Lui Por, Cheung Chi Ping and Lam Wing Kee are suspected of involvement with publisher Gui Minhai's case.

The four went missing months ago but Gui resurfaced in January, appearing on Chinese state TV to say he surrendered over a 12-year-old drunk driving case.