In a heavily secured building in Bethesda, a suburb of Washington, D.C., U.S. intelligence agencies guard a unique trove of data. The American military and its allies seized it during the war on terror in countries such as Syria and Iraq. The data comes from cell phones, USB drives, computer hard drives and paper records. Investigators call such documents "battlefield evidence."

Billions of pages of material are stored in Bethesda, according to U.S. court records. Printed out, the stack would be tens of thousands of kilometers high.

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Through the FBI, explosive datasets from this trove have made their way to Germany's Federal Criminal Police Office (BKA) in recent years. Those datasets contain secret lists of alleged fighters for Islamic State (ISIS).