Chinese regulators have unveiled an investigation into the country’s top three social media sites. Tencent’s WeChat, Sina Weibo and Baidu’s Tieba are accused of not doing enough to halt the publication and spread of illegal content.

The Cyberspace Administration of China said on Friday that it had received complaints from users. And that by failing to halt the illegal content, the platforms may in turn have broken the law. “Users are spreading violence, terror, false rumors, pornography and other hazards to national security, public safety, social order,” the CAC said in a posting on its website.

Baidu has quickly apologized and said it would cooperate with authorities. Tencent said it would assist the investigation. Sina Weibo said: “Weibo is very sorry for the negative experiences of internet users caused by harmful information.”

The past year has seen the Chinese government take an increasingly tough position on cyber sovereignty within its own borders and increase the role of the Communist Party in shaping what can and cannot be said online. It has effectively banned non-state news organizations and is currently waging a renewed campaign against virtual private networks, which are used by tens of millions of Chinese to access parts of the overseas Internet that are blocked by China’s technological defenses, sometimes known as the Great Firewall.

Last month the same three firms were ordered to conduct “cleaning and rectification.” Content deemed out of bounds included rumors about party officials and distortions of Chinese military history.

The government has made it increasingly clear that online operators must self censor and police the comments and postings made by users. That is a gigantic task. WeChat has some 940 million users globally. Sina Weibo has 350 million. Tieba’s bulletin board has 19 million sub-forums.

The CAC said that the investigation was being led by its Beijing and Guangdong branches. It will unveil further details in the coming days.