Media playback is unsupported on your device Media caption Lam Wing Kee: "You can stand up against tyranny"

A Hong Kong bookseller who went missing last year says he considered taking his own life many times while in custody in China.

Lam Wing-kee, 61, was the manager of a well-known bookstore that sold titles critical of the Chinese leadership.

Mr Lam was one of five booksellers who were imprisoned for months in cases that made international headlines.

He believes they were taken by an elite Chinese law enforcement group which targeted authors and booksellers.

One of the men, Gui Minhai, is still in custody.

Mr Lam, who was released on Tuesday, was the owner of the Causeway Bay Bookstore before it was purchased by Mr Gui in 2004.

Mr Lam told the BBC that he was accused after his arrest last October of trying to overthrow the Chinese government by mailing books to the mainland.

"They never told what the punishment for selling illegal books could be, or how long it might be. I had no idea. They could have sentenced me without consulting any sort of legal standard.

"Maybe I'd get five years in prison, or 10 years. I had no idea at all," he said.

Image copyright EPA Image caption Mr Lam's shop, Causeway Bay Books, was known for selling books about Chinese politics

He said that he had contemplated suicide in January and February but could not make any attempts because of the design of his cell.

"I was looking for a place up there to hang myself... but there wasn't," he said. He was not physically abused, he says, but endured months of solitary confinement, interrogations and psychological torture.

HK bookseller's 'toothbrush suicide watch'

Mr Lam was arrested while visiting the Chinese city of Shenzhen. He says he was taken, blindfolded, to the eastern city of Ningbo, where he was held until March.

In March, when three of his colleagues were released and returned to Hong Kong, Mr Lam was transferred to a room in the city of Shaoguan in Guangdong province, where he said he enjoyed more freedom.

China and the booksellers

1. Lui Bo, general manager. Went missing: Shenzhen, 15 October 2015 Returned: March 2016

2. Cheung Jiping, business manager. Went missing: Dongguan, 15 October Returned: March 2016

3. Gui Minhai, co-owner. Went missing: Thailand, 17 October Still missing

4. Lam Wing-kee, manager. Went missing: Shenzhen, 23 October Returned: June 2016

5. Lee Po, shareholder. Went missing: 30 December - he says from the mainland, Mr Lam says it was from Hong Kong Returned: March 2016

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Mr Lam says he was released this week on the condition that he would retrieve a hard disk filled with the names of people, mainly mainland Chinese, who had bought books from the Mighty Current publisher.

He says he has no intention of handing over the data, and has no regrets about speaking freely about what happened to him.

Special team

Who exactly was behind the operation to detain the booksellers, one of whom disappeared from Thailand and another from Hong Kong?

For months, there has been debate on whether the campaign was ordered by the highest levels of the Chinese leadership, or, perhaps, by lower levels of officialdom keen to impress the government in Beijing.

Image caption Causeway Bay Books had carried other titles, including this one with allegations about women linked to Chinese President Xi Jinping

Some believe the crackdown was prompted by the imminent publication of a book about the private life of President Xi Jinping.

Mr Lam says he does not know for sure.

But, he says, he was held by officials from an ad-hoc cross-agency law-enforcement team that can be convened only by the senior leadership in Beijing.

This elite group has roots stretching all the way back to the Cultural Revolution, when its officers were responsible for investigating Communist Party luminaries such as Liu Shaoqi.

More recently, it was believed to be responsible for investigating former security tsar Zhou Yongkang and the former Communist Party chief in Chongqing, Bo Xilai.