Senior Superintendent Li Kwai-wah of the Organized Crime and Triad Bureau said at a media briefing that in an operation in the small hours of Sunday, the police raided and searched about 22 spots and arrested 12 males and five females, aged between 21 and 53, for their suspected involvement in a string of cases in January and February in which homemade bombs were found in places including a hospital, the Shenzhen Bay border crossing and Lo Wu train station.

During the raid, the police seized three semi-finished homemade bombs containing 1.5 kilograms of explosives, three explosive devices that have not yet been connected to remote control devices, as well as about 2.6 tons of chemicals believed to be used to make bombs, Li said.

Li said the persons involved intended to use the explosives in public activities, and their making and storing the explosives and unstable semi-finished homemade bombs in commercial buildings is an extremely irresponsible acts that disregard the public safety.

Superintendent Alick Bryce McWhirter of Explosive Ordnance Disposal Bureau said at the briefing that in the raid the police cracked down "a major bomb making factory" and seized significant quantities of homemade explosives ready to be used.

"In recent months we have seen a number of powerful homemade explosives triggered by sophisticated electronics relying on radio control," he said, adding that "Hong Kong has been faced with an ongoing bombing campaign that has been both violent and indiscriminate and designed to intimidate in order to achieve political ends."