A Hong Kong bookseller has claimed a mainland official told him during his months-long detention to keep working at Causeway Bay Books when he returned to the city, to help snoop on customers buying politically sensitive books.

Lam Wing-kee, sought by mainland authorities for skipping bail but now under local police protection, mentioned the incident in a 14-page account of his detention, released to the media on Saturday.

The account states Lam wrote it from a safe house in Hong Kong on July 29.

The bookseller, who with four business associates disappeared last year and turned up on the mainland, detained for selling banned books there, said he forgot to tell media about the incident previously.

On his return to Hong Kong in June, Lam made explosive claims that he was nabbed by agents from a “central special investigative unit” when he crossed into Shenzhen last October. He claimed he had endured eight months of “mental torture”.

They wanted to find out more about Hong Kong, especially those who come to buy political books

Lam Wing-kee

In his latest account, Lam elaborated on the role of an officer surnamed Shi, who belonged to the unit and brought him to the city of Shaoguan, Guangdong province. Lam said he had to report to a police station daily after doing volunteer work at a local library.

“In Shaoguan, Shi said to me that, when I come back to Hong Kong one day, I should continue to work in the bookshop,” he wrote.

“He [said he] would contact me and I should report to him about the situation here through texts or photos,” he continued. “They wanted to find out more about Hong Kong, especially those who come to buy political books. I was to be their eyes and ears.”

Mainland authorities accuse Lam of operating a business dealing in illegal publications. That is not a crime in Hong Kong, where freedom of publication is protected.

The authorities have yet to respond to Lam’s latest claims. In July, they showed Hong Kong officials a video detailing Lam’s crimes and life in detention, asserting the bookseller’s rights had been “fully protected”. But they have kept silent as to why they did not tell Hong Kong police they were detaining him.

Pro-establishment newspapers such as Sing Tao Daily ran stories refuting Lam, including an interview with a librarian in Shaoguan who alleged Lam’s remark that he had no freedom was “a blatant lie”.

Watch: Lam Wing-kee claims ‘forced confessions’

Earlier this year, an unknown investor bought shares in the publishing house that owned Causeway Bay Books, but the shop remains closed. “Why do those people want to buy the bookshop and let it be vacant?” Lam wrote in his latest statement. “It’s clear those people have the aim of using the bookshop as their watch post to keep Hongkongers under surveillance.”

The store was not cheap, Lam added in the statement, noting its rent of HK$40,000 per month.