Woo, 75, who assisted Lee in managing the bookstore specialising in publications critical of the Chinese Communist Party, told the Post that the disappearances were all too familiar.

“I was taken away during my visit to a dentist in Shenzhen,” an agitated Woo recalled.

He remembered clearly that the agents snatched him in March 2012 and interrogated him for 38 hours.

“A group of security agents took turns asking me for information on who’s who in the Hong Kong literary circles and they handed me of a list of pen names and asked me to identify them,” the author of numerous biographies said.

“I told them if I had known I wouldn’t have worked so hard as a writer to make ends meet.”

He recalled that a senior officer identified himself as the duty officer responsible for the case of Ching Cheong, the reporter for Singapore’s Straits Times who was detained in Shenzhen and later jailed on espionage charges.

“The officer said he could easily accord me the same prison term like Ching if I didn’t co-operate,” he said. Woo clarified that the agents treated him in a “mild” manner. He was not yelled at or beaten up, he said.

He was not worried about his own safety, he said, and saw the need to speak up.

“The real story will be told when Lee Po returns to Hong Kong,” he added.

That mystery has deepened as it now turns out that Lee sent an email to Gui’s daughter, Angela, on November 10, saying: “I write to you concerning the whereabouts of Michael (Gui’s English name).

“I wonder if you have known that he has been missing for more than 20 days, we fear that he was taken by special agents from China for political reasons.”

“We last talked to Michael by email on 15 October, and after that day, nobody could contact him. He was then staying in his apartment in Thailand. According to [Gui’s family]’s words told by the watchman of the building, he left the apartment with several men who claimed to be his friends,” the email reads.

“It’s very little we can do to help Michael because we are not his next of kin. I then think of you, perhaps you can do something, and there are a lot of Michael’s friends [who] are ready to help if you need them. Do tell me what you think and what you want us to do.”

The email contradicted Lee’s story, which he gave weeks after he disappeared. In a letter to his wife, Sophie Choi Ka-ping, Lee blamed Gui for his predicament, describing him as a “morally unacceptable person” who had a “complicated personal history”.

In October, Gui went missing under mysterious circumstances in Pattaya, Thailand. In the same month, Lui Por, Cheung Chi-ping and Lam Wing-kee also vanished while on the mainland.

At the end of December, Lee went missing from Hong Kong, with no record of him crossing the border.

Their disappearances led to fears they had been kidnapped by mainland agents because the publishing house and bookstore they ran specialised in publications critical of the Chinese Communist Party.

Throughout the episode, the Hong Kong government has maintained that there is no evidence of cross-border law enforcement related to the disappearances.

In January, Gui was paraded on state television, claiming he had surrendered himself to the mainland authorities over a 2004 drink-driving accident in Ningbo (寧波), Zhejiang (浙江) province.

He was later accused of ordering his associates to deliver about 4,000 banned books across the border since October 2014.

“I definitely think that he was pressured to say the things that he said. Because I have never heard of these things, these claims at all,” Angela Gui said from the UK.

The 22-year-old said her father’s “confession” on CCTV “definitely seems scripted”, and she hoped to see him as soon as possible.

“It isn’t a plan as much as it is a hope at the moment because there is so much going on with the case.

“So I can’t make any concrete plans for the time being. I, of course, hope to be able to see my dad as soon as possible. It would be absolutely fantastic to go and meet him,” she said.