Werfalli has thus far evaded justice. But human-rights activists still hail the case as a breakthrough for a powerful new tool: online open-source investigations. Even in no-go combat zones, war crimes and other abuses often leave behind an information trail. By piecing together information that becomes publicly accessible on social media and other sites, internet users can hold the perpetrators accountable—that is, unless algorithms developed by the tech giants expunge the evidence first.

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Shortly after the Werfalli arrest warrant was issued, Hadi Al Khatib, a Syrian-born open-source investigator based in Berlin, noticed something that distressed him: User-generated videos depicting firsthand accounts from the war in Syria were vanishing from the internet by the thousands. Khatib is the founder of the Syrian Archive, a collective of activists that, since 2014, has been scouring for digital materials posted by people left behind in Syria’s war zone. The Syrian Archive’s aim is “to build a kind of visual documentation relating to human-rights violations and other crimes committed by all sides during the eight-year-old conflict,” Khatib said in an interview.

In the late summer of 2017, Khatib and his colleagues were systematically building a case against the regime of Bashar al-Assad in much the same way ICC investigators pursued Werfalli. They had amassed scores and scores of citizens’ accounts, including video and photos that purportedly showed Assad was targeting hospitals and medical clinics in bombing campaigns. “We were collecting, archiving, and geolocating evidence, doing all sorts of verification for the case,” Khatib recalled. “Then one day we noticed that all the videos that we had been going through, all of a sudden, all of them were gone.”

It wasn’t a sophisticated hack attack by pro-Assad forces that wiped out their work. It was the ruthlessly efficient work of machine-learning algorithms deployed by social networks, particularly YouTube and Facebook.

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With some reluctance, technology companies in Silicon Valley have taken on the role of prosecutors, judges, and juries in decisions about which words and images should be banished from the public’s sight. Lately, tech companies have become almost as skilled at muzzling speech as they are at enabling it. This hasn’t gone unnoticed by government entities that are keen to transform social networks into listening posts. Government, in effect, is “subcontracting” social-media platforms to be its eyes and ears on all kinds of content it deems objectionable, says Fionnuala Ní Aoláin, a law professor and special rapporteur for the United Nations Human Rights Council.

But some of what governments ask tech companies to do, such as suppressing violent content, cuts against other legitimate goals, such as bringing warlords and dictators to justice. Balancing these priorities is hard enough when humans are making judgments in accordance with established legal norms. In contrast, tech giants operate largely in the dark. They are governed by opaque terms-of-service policies that, more and more, are enforced by artificial-intelligence tools developed in-house with little to no input from the public. “We don’t even know what goes into the algorithms, what kind of in-built biases and structures there are,” Ní Aoláin said in an interview.