HONG KONG (AP) — Mainland Chinese authorities confirm they've detained three missing Hong Kong booksellers for an investigation into unspecified criminal activity, shedding more light on a case that's gripped residents with fear that Beijing is tightening its hold on the city.

Hong Kong police said late Thursday they were told by police in neighboring Guangdong province about the men, who are among five people linked to a publishing house specializing in politically sensitive titles banned on the mainland to have vanished in recent months.

It's the latest development in a case that's sparked international concern that Beijing is backtracking on a promise it made when it took over from Britain nearly 20 years ago to let the city retain a high degree of control over its own affairs.

The five are associated with publishing firm Mighty Current Media and its retail outlet Causeway Bay Bookshop. The company's books on political scandals and intrigue involving China's communist leaders are popular with mainland Chinese visitors to Hong Kong.

The letter from Guangdong's public security department was the first time that mainland authorities had acknowledged holding Lui Por, Cheung Chi Ping and Lam Wing Kee, who are shareholders or employees of the company.

It said the three are suspected of involvement in a case involving a person surnamed Gui, an apparent reference to Mighty Current publisher Gui Minhai.

The four went missing in October but Gui resurfaced in January, making a tearful appearance on Chinese state TV to say he surrendered over a 12-year-old fatal drunk driving case.

The three are suspected of being "involved in illegal activities on the mainland," Hong Kong police said, citing a letter from Guangdong Provincial Security Department's Interpol liaison office.

"Criminal compulsory measures were imposed on them and they were under investigation," it said. No details on the alleged crimes or their specific whereabouts were disclosed.

Gui, a Swedish national, disappeared from his holiday home in Pattaya, Thailand, while the three others went missing on the mainland.

Hong Kong police also said they received a handwritten letter from the fifth missing person, editor Lee Bo, in which he purportedly rejected a request to meet with them. Lee, a British citizen, disappeared on Dec. 30 and many suspect he was abducted by mainland Chinese security agents operating in Hong Kong, which would be a breach of the "one country, two systems" principle Beijing agreed to when it took control of the city from Britain in 1997.

The European Parliament called Thursday for the five to be immediately released, joining British, American and Swedish officials who have raised concern about the case.

The latest announcement appears unlikely to quell suspicions by human rights activists and pro-democracy groups that Lee and Gui's statements were made voluntarily.

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