A Hong Kong bookseller who claims to have been abducted by Chinese special forces has said his interrogators wanted to find out the names of Communist party officials who might have been sources for the banned books he distributed on the mainland.

Lam Wing-kee said his interrogators were particularly interested in who had supplied information for the book Xi Jinping’s Dream of a 20-year Rule, published last year by Gui Minhai, who later went missing.

Banned on the mainland, the book accuses the Chinese president of trying to extend his rule beyond the 10 years normally served by Communist party leaders, in a similar way to how Vladimir Putin has been able to stay in power in Russia.

Lam said he believed the crackdown was aimed at rooting out a clique within the party suspected of seeking to undermine Xi.

“It looks very likely that that is the case,” Lam said in an interview with the Guardian on Monday. “They were looking for sources of the books that had remained confidential.” Last week Lam, 61, became the first of five detained booksellers to confirm he had been forcibly abducted and to provide details.

After months of solitary confinement on the mainland, he said his interrogators had allowed him to return to Hong Kong this month to retrieve a hard drive containing names of mainland citizens who had purchased his books. Instead of going through with this arrangement, he decided to go public and stay in Hong Kong, despite potential consequences for his girlfriend on the mainland.

Lam said he had agonised over the decision, fearing reprisals against his partner and also relatives of other booksellers. But he said he felt compelled to get the story out and could not bear the thought returning to China to face more interrogations and confinement.

“They accused me of creating rumours of top leaders and said I posed a big threat to society. They said that people like me are deserving of the most severe treatment,” he said. “They told me they could keep me for 10 to 20 years if I did not cooperate. I did not know where I was. It was very scary. Even now I get scared when I think about it.”

Lam’s revelations have heightened fears that Hong Kong residents and Chinese-born critics abroad could be abducted wherever they live. Those fears first emerged when Gui, a Chinese-born Swedish citizen, disappeared from Thailand on 17 October and later ended up in detention on the Chinese mainland.

Jerome Cohen, a China expert and professor of law at New York University, said Lam’s case “offers classic instruction about the arbitrary power of the Chinese police to detain and torture any of us within China and to reach beyond China borders”.

He said Lam’s assertion that he was interrogated by a Chinese special investigative group – the same type of unit as has been used to take down senior party officials such as Zhou Yongkang, a senior politburo member convicted of corruption – was also instructive. “He has confirmed that this case is not the product of some out-of-control local officials in southern China but emanates from the party centre,” Cohen said.

William Nee, a China researcher for Amnesty International, said: “Lam’s allegations would seem to indicate that this is being ordered from almost the highest levels of Chinese government.”

Lam said he learned that special forces were involved soon after he was stopped on 24 October while trying to enter the mainland through an immigration point in Shenzhen. He knew one of the police officers from a previous encounter. He tried joking with him, he said, but the officer made a grave expression and said he was part of special investigations unit. “‘You’ve made a big mistake,” Lam recalled him saying. “We are here to interrogate you.”

Lam said he was taken blindfolded to a detention centre that he later learned was in Ningbo, near Shanghai. There his interrogators grilled him, daily at first and then every week or so as the months passed. That was the only time he was allowed to leave what he said was a padded cell, designed to prevent detainees from killing themselves.

“From morning to night I couldn’t speak to anyone,” he said. “It was a form of psychological torture.” By January, Lam said, he was thinking about suicide but could not find the means.

He said his chance for freedom came when interrogators recognised that he could not provide valuable information on inside sources for the books. They then turned their interest to obtaining his customer list, which allowed him to go back to Hong Kong to retrieve the hard drive.

Lam said he had made up his mind to go public after seeing a video of 6,000 protesters in Hong Kong who came out in support of the booksellers. “After watching that video, I thought: this is not just about a few people in Hong Kong, but all of Hong Kong. If I didn’t speak out and neither did my colleagues, no one would know what has happened.”