Sri Lankan soldiers on Monday inspect the damage inside St. Sebastian’s Church, where a bomb blast took place in Negombo, Sri Lanka. (Tharaka Basnayaka/Bloomberg)

EXPLOSIONS IN Sri Lanka killed at least 290 people and injured more than 500 on Sunday, an unconscionable attack on Christian worshipers on their most sacred day. The story behind the tragedy is still murky, but officials have taken one drastic step to prevent false tales from spreading: They shut down access to several of Sri Lanka’s most popular social media services.

This weekend’s bombings, which officials say were carried out by a local Islamist militant group, were a heartbreaking blow to a nation still healing from a decades-long civil war in a region that has lately been beset by religious discord. In this context, Sri Lanka’s choice to block Facebook, WhatsApp and other platforms might be understandable. The country was suddenly rocked by mass violence, and misinformation on these services has a pattern of fomenting even greater unrest.

The decision, however, sets an alarming precedent.

Sri Lanka’s move against social media is the latest rebuke of platforms once seen as a great hope of the democratic world. Today, governments are more likely to view the sites as breeding grounds for lies and propaganda that cannot be trusted to act against dangerous content when it matters most. Sri Lanka has experience: One year ago, rumors on Facebook mobilized members of the Buddhist majority against Muslims. The firm eventually vowed to hire more moderators and improve enforcement — but only after the government shut down access to the site, and only after a man burned to death.

Now, Sri Lanka has shut down access without any evidence that rumors or rhetoric on the site led to harm. The justification, presumably, is that once evidence has emerged, it is already too late. But a moratorium on Facebook and other essential services in Sri Lanka is also a moratorium on easy communication. Friends and families cannot tell each other whether they are safe. Anyone seeking accurate information about the attacks may find themselves similarly stymied; because controls on traditional media in Sri Lanka are tight, Facebook and sites like it play a valuable role. Meanwhile, technologically sophisticated users, including many bad actors, will find workarounds to reach blacked-out websites anyway.

These concerns underscore a larger worry: that one state blocking services to keep civilians safe after a terrorist attack will serve as a pretext for other states, not to protect their people, but to deprive them of their ability to organize and protest. Iran restricted access to the Telegram app last year amid anti-government demonstrations. Turkey, Egypt and Zimbabwe play at the same game. Facebook and sites like it will have to earn back the globe’s trust, and countries that still value free expression must help craft regimes that balance safety and freedom of expression. Otherwise, those less friendly to democracy are bound to use the former as an excuse to trample on the latter.