Mystery surrounds the whereabouts of four men working for a Hong Kong company publishing books on sensitive mainland issues - amid fears they may have been detained by mainland authorities. The four include Gui Minhai, a mainland-born Swedish national and co-owner of Mighty Current publishing company, which has published about 80 books on China since its establishment in 2012. The company also runs a book shop in Causeway Bay. Its books mostly cover mainland politics, power struggles and scandals involving officials. A colleague and co-owner, surnamed Lee, said Gui's whereabouts were unknown since he went on holiday in Pattaya in Thailand in the middle of last month. Gui was last heard of when he sent an email on October 15 to printers asking them to get ready for a new book. Since then, Gui could not be reached, Lee said. The other three Hong Kong residents went missing after they visited Shenzhen separately late last month, Lee said. The three are Lui Bo, general manager of Mighty Current, business manager Cheung Jiping and Lam Wing-kei, who is manager of the bookstore. Lee said Lam telephoned his wife, who lives in Hong Kong, last Friday, saying he was "alright" but would stay away for a while. Lam did not mention where he was at the time, according to Lee. Gui's wife lives in Germany, while the wives of Lui and Cheung live on the mainland. Lee said he and Lam's wife sought police help last week. Police said they had received a report from Lam's wife last Thursday and that she had told them the following day that she had been in contact with her husband. Lee said he feared the four might have been detained by mainland authorities as the book Gui was sending to print probably touched on sensitive issues. "I suspect all of them were detained. All four went missing at the same time," Lee said. He added that phone calls to them were either unanswered or went dead. William Nee, a China researcher with human rights campaign group Amnesty International Hong Kong, said the disappearances came in the context of increased control over mainland publications. "If the claims are true, it would be a very worrying sign that the mainland exerts pressure on freedom of expression in Hong Kong," Nee said. In late 2013, Hong Kong-based publisher Yiu Man-tin, also known as Yao Wentian, was arrested and detained in Shenzhen. At the time, he was working on a dissident's book about President Xi Jinping . Yiu was detained with seven bottles of undeclared paint he brought from Hong Kong to Shenzhen. His relatives believed Yiu was "lured" to Shenzhen on the pretence of delivering the paint to a long-time friend. In May last year, Yiu, 73, was sentenced to 10 years in jail by a Shenzhen court for "smuggling ordinary goods".