Pro-Beijing lawmaker and former Hong Kong security chief Regina Ip urged the city's government to "seriously handle the matter" as mainland officials are not entitled to carry out official duties in Hong Kong.
"The Hong Kong government needs to get to the bottom of this very quickly, give a full and proper account to the people of Hong Kong, and put people's minds at ease", she said.
The latest missing bookseller is Lee Bo, believed kidnapped on December 30.
Anonymous has also posted a message on Sunday night, claiming that its Hong Kong division was in the process of working with overseas teams on the planned attacks.
All five are co-owners or employees of Mighty Current, a publisher known for books about China's leadership, many of which are banned on the mainland.
Lee Cheuk-yan, a pro-democracy legislator, said Chinese law enforcement agencies had yet to respond to requests for information from Hong Kong officials. "Here, we call for all parties fighting for democracy in Taiwan, China, Hong Kong and Macau to defend our rights to freedom... we should continue to fulfil our duties as a global citizen and pay close attention to the state of human rights and freedom in China, Hong Kong and Macau".
Five people associated with Hong Kong publisher Mighty Current and its Causeway Bay Bookstore have vanished in recent months.
When Lee vanished last Wednesday, he reportedly did not have a travel permit for mainland China with him, triggering speculation he did not plan to go there and that Chinese security agents abducted him. Ng, though, provided no evidence.
"It would not be acceptable for someone to be spirited out of Hong Kong in order to face charges in a different jurisdiction", Hammond said. But some say their disappearances are tied to a book purporting to detail the marital affairs of President Xi Jinping. "Those books have through various channels entered into the mainland and have become a source of certain political rumors, which have caused some evil influence".
Formerly a British colony, Hong Kong was handed over to China in 1997 and is now a "special administrative region of China", BBC explains. Last year, thousands of protesters occupied areas downtown to demand democratic elections, which infuriated leaders in Beijing.
The intent, they say, is to tame the public outcry that has been sparked by his disappearance.
"We have been very disappointed with Britain's overall reaction to attempt to pull back from the Joint Declaration and the Basic Law", said Mrs Chan.
Copyright 2016 NPR. To see more, visit http://www.npr.org/. As for the titles he sells, "50% of our books are not allowed in China".
FRANK LANGFITT, BYLINE: Good morning, Renee. "We are monitoring all the cases closely", a statement read. He works at a place called Causeway Bay Books. Well, last week he disappeared. However, under Hong Kong law, only the subject of a missing person report can cancel it.
Leung also reiterated that the freedom of the press, publications and expression are legally protected in Hong Kong. "The law protects the rights, including the freedom and safety of everybody in Hong Kong". Is this all getting so much attention because it is Hong Kong? There's free speech and rule of law there. Mrs Lee's response reflects a typical assumption of Hongkongers - that you could criticise the Communist Party as long as you are in Hong Kong.
"Lee Bo's case is a game changer".
MONTAGNE: That does sound a little scary.
And now, it appears that the crackdown is seeping into Hong Kong.