HONG KONG — A Hong Kong bookseller whose disappearance in December provoked protests and concern over the state of media freedom here returned to this semiautonomous city from the Chinese mainland on Thursday, the Hong Kong police said.

The bookseller, Lee Bo, was one of five people associated with the company Mighty Current Media to have disappeared, only to emerge in mainland China, apparently in detention. The company published gossipy books on elite politics in China, and the detentions were seen by many in Hong Kong as an effort to clamp down on media voices critical of the Chinese Communist Party.

The case of Mr. Lee, also known as Lee Po, was particularly troubling to civil society advocates because he disappeared from the streets of Hong Kong, and it is still unclear how he crossed a border with Guangdong Province on the mainland. He is widely suspected of having been taken by mainland security officers, although he told the Hong Kong authorities that he had “returned to the mainland by his own means voluntarily and it was not an abduction,” the police said in a statement.