In an early morning raid involving hundreds of officers, Hong Kong national security police arrested five directors of the pro-democracy Apple Daily newspaper.
Thursday's raid is the latest blow to media tycoon Jimmy Lai who owns the tabloid and is a staunch Beijing critic.
Officers entered the tabloid with a warrant to seize journalistic materials.
They were later seen sitting at computers in the newsroom and sifting through reporters' notebooks.
It's the first time authorities have cited media articles as potentially violating the national security law.
The legislation, which came in last year, is not retrospective but can use actions from before its implementation as evidence.
Senior superintendent Li Kwai-wah said the tabloid published dozens of reports dating back to 2019 that "incited foreign countries to impose sanctions".
Lai has been in detention since December and was denied bail under the security law.
He's serving several sentences for taking part in unauthorised rallies in 2019 and faces allegations of national security offences.
Under that same law, Lai's assets have been frozen.