HONG KONG - For over eight months, bookseller Lam Wing Kee was interrogated 30 to 40 times by mainland authorities. Each session lasted 30 to 40 minutes.

In the end, the interrogation zeroed in on who were the sources that provided the material for books critical of Chinese President Xi Jinping that sold so well among mainlanders, Mr Lam told The Straits Times on Sunday (June 19).

Mr Lam and four of his colleagues at publishing firm Mighty Currents and Causeway Bay Books store, which specialise in salacious gossip about China's leaders, went missing late last year. They resurfaced on the mainland only early this year.

By Mr Lam's account, he was allowed by mainland authorities to return to Hong Kong for two days last week on the condition that he returned to the mainland on Thursday.

But he decided to stay in Hong Kong and held a press conference where he told reporters about his and his colleagues' abductions and "rehearsed" confessions.

The latest twist to the saga of the booksellers has further deepened Hong Kongers' distrust of the central government. Some 6,000 protesters demanding freedom for Hong Kong took to the streets on Saturday.

Some observers have raised the possibility that the crackdown came because Mr Xi wanted to silence rivals who might have been the ones disseminating the information. Mr Lam said on Sunday he agreed with this interpretation.

"Perhaps our sales volume has also increased too significantly in recent years, resulting in adverse effects on the mainland. Since 2013, we have started mailing books to the mainland. Besides, with increasing demand, some people have been smuggling our books to the mainland," said Mr Lam.

"But I believe what they really want to know is the source of the content for those books."