Hong Kong's National Security Department has arrested a 43-year-old local man suspected of sending threatening letters to multiple media organizations and operating a website supporting activities that could endanger national security, police revealed on Friday.

The case came to light in June when several media companies reported receiving extortion letters warning them to publish a specific article or face arson attacks on their offices, according to Superintendent Chow Hok-yin of the Police National Security Department.

The article in question promoted a website allegedly selling commemorative items to raise funds for two concerning purposes: supporting fugitives wanted under the National Security Law who have fled overseas, and backing campaigns urging foreign governments to sanction Hong Kong officials and judges, said Chow.

Authorities determined both the threats and website content potentially violated national security, prompting the National Security Department to take over the investigation.

Police identified the suspect through digital forensic work and traced the website, which was hosted on an overseas platform.

Officers arrested the 43-year-old suspect Tse in Fanling yesterday. A subsequent search of his residence uncovered computer files containing website images and data, along with detailed lists of local media organizations - including those that had reported the threats - and their office addresses.

The suspect remains in custody as investigations continue.

(Marco Lam)