Five hundred Hong Kong police officers raided the offices of pro-democracy tabloid Apple Daily on Thursday. Five of the outlet’s top executives were arrested, its accounts were frozen, and the public was warned not to repost its articles online, in what is the strongest blow yet to the territory’s media freedoms.

The strike marks the first time that authorities have accused specific media articles of violating the controversial National Security Law (NSL) imposed by Beijing last summer, as well as one of the most aggressive deployments of the new legislation so far.

A police squadron of around 500 entered Apple Daily’s newsroom with a warrant they said afforded them the power to “seize journalistic materials” and rifle through reporters’ phones and computers in service of gathering evidence.

At around 7 a.m. local time, police also arrested five of the newspaper’s top executives at their homes, on the grounds that they had violated the NSL by “colluding with a foreign country or external elements to endanger national security.”

Those apprehended were the paper’s Editor-in-Chief Ryan Law, CEO Cheung Kim-hung, COO Royston Chow, associate publisher Chan Pui-man, and digital platform director Cheung Chi-wai, according to Apple Daily itself. The Hong Kong police do not issue names of those it apprehends.

Apple Daily’s owner, the 73-year-old outspoken pro-democracy figure Jimmy Lai, was jailed in December for his participation in 2019 anti-government protests and denied bail. Last month, he was sentenced to a further 14 months in prison for his participation in a different “unauthorized assembly” that same year. His combined sentence means he will be behind bars for a total of 20 months.

Authorities stated that more than 30 Apple Daily reports dating back to 2019 — before the NSL was established — had potentially violated the law. Although the law is not retrospective, prior incidents can be used as evidence.

Hong Kong’s senior superintendent Li Kwai-wah told the press on Thursday that it was “very straightforward” why the articles were under fire. They were “inciting, requesting foreign countries to impose sanctions on Hong Kong and the People’s Republic of China.”

Li cautioned readers from sharing Apple Daily articles, warning them “not to invite suspicion.”

The U.S. last year imposed sanctions on Chinese and Hong Kong officials for suppressing the 2019 pro-democracy protests, among other restrictions.

Mark Simon, one of Lai’s advisors currently outside of Hong Kong, told Reuters: “This is a blatant attack on the editorial side of Apple Daily.”

Before markets opened on Thursday, Lai’s Hong Kong-listed company Next Digital, which published the tabloid, announced that it had suspended trading in shares, without providing further details.

Police have also frozen $2.32 million of assets owned by three firms tied to Apple Daily. Last month, authorities also ordered Lai’s shares in Next Digital to be frozen.

This is the second raid on Apple Daily. Last summer, more than 100 police swarmed its offices and arrested 10 people, including Lai and his sons.

The NSL gives authorities the power to condemn any act considered subversion, secessionism, terrorism or collusion with foreign forces with a sentence of up to life in prison. In the first year since its implementation, it has been used to arrest 112 people and charge 57.

Asked by Reuters how long Apple Daily might be able to continue publication, Simon said: “It’s not up to us. It’s up to them. There’s 100 police officers in our newsroom. They decide, not us.”