Hong Kong police have arrested five people alleged to be involved in a bombing plot. National security police officials said Tuesday that they have placed four males and one female in custody over suspicion of "conspiring to plan terrorist activities."

The arrests were made under the recently imposed national security law - legislation that was part of China's attempt to address the months of often violent protests in the former British colony. Officials said they had reason to believe that those that were arrested were linked to a possible bombing plot.

Police said that the investigation into the matter is continuing and they may conduct further arrests in relation to the case. Police said they had arrested nine people July 6, including six high school students, who were linked to the alleged bombing plot. Officials said the group was planning to use explosives to destroy transport facilities, including train stations and tunnels.

 Senior Superintendent Li Kwai-wah said the nine people arrested were identified to be members of a pro-democracy group called Returning Valiant. Police said they found chemicals in a makeshift laboratory inside a hotel within the city. Police said the chemicals could be used to manufacture triacetone triperoxide - or TATP - a type of explosive used by terrorists in Europe and other countries.

"[The group] wanted to attack some of the public facilities in Hong Kong, including the Cross-Harbor Tunnel, railways, courtrooms and they even wanted to lay bombs in the rubbish bin on the street, with a view to maximizing the damage caused to the society," Li said.

Authorities said they have frozen the assets of all those arrested. Six out of the nine people had since been released on bail, while the remaining three are set to appear in court in September.

Tensions in Hong Kong continue to rise because of China's tightening crackdown on political speech. Last month, a worker stabbed a police officer patrolling a busy shopping district before taking his own life. People who expressed their support of the man were quickly shut down by authorities while also warning the public against speech that "attempts to romanticize or glorify despicable acts."