Sri Lanka blocked several social media networks in the wake of terrorist attacks on Sunday, including Facebook and the messaging service WhatsApp. The extraordinary step reflects growing global concern, particularly among governments, about the capacity of American-owned networks to spin up violence.
YouTube, Instagram, Snapchat and Viber were also inaccessible, according to internet monitoring groups.
“This was a unilateral decision,” said Harindra Dassanayake, a presidential adviser in Sri Lanka.
Officials blocked the platforms, he said, out of fear that misinformation about the attacks and hate speech could spread, provoking violence.
Sri Lankan officials have a troubled relationship with social media, which many in the country credit with helping bring democracy after years of civil war, but also accuse of fomenting racial fear and hatred.
Last year, the government briefly blocked social networks after viral rumors and calls to violence, circulating largely on Facebook, appeared to provoke a wave of anti-Muslim riots and lynchings.
Government officials had repeatedly warned Facebook, which also owns Instagram and WhatsApp, that the posts could lead to violence. Company officials largely failed to respond until the government shut down access, after which they promised to hire more moderators and improve communication.
That episode left officials wary of the social network.
Other countries have periodically blocked social media during spasms of violence linked to the platforms. India first blocked access to Facebook in 2012 amid riots linked to false information on the platform. Last summer, viral rumors on WhatsApp in the country appeared to incite a series of mob attacks.
But Sri Lanka’s decision to block social media on Sunday was unusual. The government shut down access before any social-media-inspired violence was known to have taken place.
The move suggested not just officials’ worries about social media’s risk to public safety at moments of national tension, but also their distrust in the companies’ ability to manage the platforms responsible.
That reflects a global, and growing, wariness toward social platforms and the giant American corporations that run them, said Ivan Sigal, the executive director of Global Voices, a digital advocacy and journalism organization.
“A few years ago, this would have been outrageous,” Mr. Sigal said.
That digital rights and press freedom advocates might now sympathize, if not agree, with Sri Lanka’s action “is a damning indictment” of companies that once portrayed the platforms as vehicles for liberation, he said. He called it “a signal of the lack of trust that’s built up around their practices.”
“It’s no longer the presumption that they are effective, benevolent or possibly positive,” Mr. Sigal said. “That was true once upon a time.”
A growing body of research has linked social media to religious and racial violence.
Social media platforms build their businesses on sophisticated algorithms that serve up content that will keep users engaged. This favors posts that tap into negative, primal emotions like anger and fear, studies suggest.
Misinformation is rampant, though Facebook and YouTube say they are working to reduce it.
That can be deadly in countries with recent histories of community violence. Last year, United Nations officials said Facebook had played a “determining role” in Myanmar in whipping up genocide; thousands were killed.
Developing markets, particularly in Asia, represent the future of Facebook’s growth-focused business model. But Sri Lanka is not the only country whose government has an increasingly tenuous relationship with the social networks.
European officials are putting more pressure on the companies to address what is seen as a tendency to drive dangerous speech. Germany imposed rigorous legal restrictions last year mandating that the networks rapidly remove hate speech. A recent study in Germany found that higher rates of Facebook usage corresponded with more attacks on refugees.
All but the most authoritarian governments have typically been wary of blocking social media access, as the services are popular.
And in any case, blocks are only somewhat effective, because users can download apps, known as V.P.N.s or virtual private networks, to circumvent them. A study on last year’s Sri Lankan block, conducted by Yudhanjaya Wijeratne, a Sri Lankan researcher, found that use of social networks dropped by only about half.
Social media blocks, moreover, disproportionately affect poorer and more vulnerable users, who cannot get access to V.P.N.s, Mr. Sigal said.
In Sri Lanka, social networks are the main communication tool for many. When they are blocked, it becomes more difficult for families to stay in touch or determine whether loved ones were harmed in attacks like those on Sunday, said Kate Cronin-Furman, a scholar at University College London.
Minority activists in Sri Lanka are already worried that the government has set a precedent for blocking communications during a security incident, Ms. Cronin-Furman added.
Mr. Dassanayke, the Sri Lankan official, said that the government had no firm plans for when to restore access to the services. He said it would consider negotiating with the social media companies.
A Facebook spokeswoman said the company was committed to maintaining access in Sri Lanka and removing any content that violates its standards.
In an interview last year, the company’s vice president for policy solutions, Richard Allan, said that Facebook might seem powerful to small governments like the one in Sri Lanka, but that this was not really the case.
“They have the ultimate weapon, which is the off switch,” he said.