Hong Kong police arrested a columnist for the pro-democracy Apple Daily newspaper under the city’s national security law in an operation that is still ongoing, the latest blow to a popular tabloid that is facing an imminent shutdown under pressure from the government.

Police on Wednesday arrested a 55-year-old man for allegedly colluding with foreign forces to endanger national security. While the police statement didn’t mention any names or publications, local media including the Oriental Daily reported the man is an Apple Daily columnist who writes social commentary under the name Li Ping.

The operation is ongoing and the police wouldn’t rule out additional arrests, a member of the police’s public relations team told Bloomberg News.