Mainland China officials have confirmed that they detained the three missing Hong Kong booksellers over an investigation into an unspecified criminal activity. The case has gripped Hong Kong where residents are speculating if China is trying to assert its influence in the region and attempting to clamp down on anti-Beijing sentiments.

Police officials in Hong Kong announced late Thursday that they received a letter from Chinese police officials in the neighboring Guangdong province about the three men, of the total five missing since October, the Associated Press (AP) reported. The men were among those linked to Mighty Current Media and its retail outlet Causeway Bay Bookshop in Hong Kong that published material critical of China. The publishing house’s books on political scandals and intrigue regarding leaders of the Communist country have been popular among Chinese visitors to Hong Kong.

The letter from Guangdong's public security department was the first confirmation of the speculation that the publishing house's Lui Por, Cheung Chi Ping and Lam Wing Kee were detained by Chinese authorities. The letter said that the three men were suspected to have been involved in a case linked to a person named Gui and that they were “involved in illegal activities on the mainland,” the AP reported. 

The letter also said: “Criminal compulsory measures were imposed on them and they were under investigation,” without disclosing more information.

The European Parliament Friday called for the immediate release of the men. “The resolution calls for their immediate safe release. It also calls for the immediate release of all other persons arbitrarily arrested for exercising their rights to freedom of expression and publication in Hong Kong,” it said, according to Reuters.

Incidentally, a publisher of the Mighty Current is also named Gui Minhai, a Swedish national who went missing from a holiday in Pattaya, Thailand, over there months ago. He was set to publish a book on Chinese President Xi Jinping’s “love affairs,” before his disappearance. In January, he made an appearance on China’s state network and surrendered over a fatal drunk-driving case that happened 12 years ago. Hours after the address, his daughter Angela said that she received a message on Skype from Gui asking her not to disclose any details about him.

Angela described Gui's confession as “ridiculous” and contrived, while Swedish authorities have been demanding Beijing to show “openness” in the case.

Last month, authorities in Hong Kong also received a letter from missing editor Lee Bo, in which he said he could not meet the Hong Kong officials as he was on the mainland assisting authorities in an investigation. Lee disappeared on Dec. 30 and many suspect that he was taken to mainland by Chinese authorities in Hong Kong. Although Lee’s wife confirmed that the handwriting in the letter was indeed his, several others suspected that he may have been forced to write the letter.

Lee had raised an alarm over his colleagues' disappearance in October, but in the letters, Lee said that he “voluntarily” went to the mainland to help Chinese officials in an investigation and blamed Gui for getting him involved in it. Lee is believed to be a British passport holder and the U.K. government has sent diplomatic requests for more information on him.

Also, if speculations that Lee was taken from Hong Kong by Chinese authorities are true, it would constitute a violation of the "one country, two systems" formula, which allows a degree of autonomy and freedom, including freedom of speech, to Hong Kong, which was returned to China from British rule in 1997.

The other three men, whose whereabouts were confirmed Thursday, went missing on the mainland and there also have been suspicions over the confessions from Lee and Gui.

"The latest official disclosures about the last three missing book publishers are anything but satisfactory," William Nee of Amnesty International said in a statement, according to the AP, adding: “The Chinese authorities need to end their smoke and mirrors strategy and come clean with a full and proper explanation.”