Jimmy Lai, a media tycoon and critic of the Chinese Communist Party, was arrested on August 10 on charges of "collusion with a foreign country".
Apple Daily, a fiercely pro-democracy newspaper that regularly takes on the Hong Kong government and the Chinese leadership.
He is denounced by Chinese officials, pro-Beijing news outlets in Hong Kong, and China's state-run news media.
According to Apple Daily, 72-year-old Lai was being investigated on charges of partnering with a foreign country.
Besides Lai, his two sons have also been arrested on charges of violating company business code.
The authorities are probing Lai's private investments as well.
The draconian law is aimed at crushing dissent in the erstwhile British colony which saw massive pro-democracy protests last year.
The legislation came into effect on July 1, 2020.