Although three other booksellers connected with Mighty Current Media revealed little about their detentions after they were released this year, Mr. Lam spoke in detail about being taken into custody at Hong Kong’s border with the mainland and spending months in solitary confinement. He also told a news conference how the mainland authorities had tried to pressure him into giving up a list of bookstore customers.

Rimsky Yuen, Hong Kong’s secretary for justice, and Lai Tung-kwok, the secretary for security, traveled to Beijing for meetings on Tuesday with Guo Shengkun, the Chinese minister of public security, about the booksellers’ case and the mechanism for informing Hong Kong about the detention of residents in the mainland.

The Beijing authorities said that Mr. Lam had admitted that he and others had collected more than 400,000 renminbi, or about $60,000, selling banned books to customers in mainland China, and that he violated the conditions of his parole by not returning there, the Hong Kong public broadcaster RTHK reported. If he does not return, he could face harsher penalties, the mainland authorities said.

The mainland authorities also released a video of Mr. Lam confessing, RTHK reported.

Hong Kong does not have an extradition agreement with the mainland. Mr. Lam said during his news conference that he had no plans to return there.

Since his revelations last month, Mr. Lam has been praised by members of Hong Kong’s pro-democracy camp for standing up to pressure from the mainland authorities. But he has also been under attack from some former colleagues and pro-Beijing news outlets questioning his story.