YouTube videos of police beatings on American streets. A widely circulated internet hoax about Muslim men in Michigan collecting welfare for multiple wives. A local news story about two veterans brutally mugged on a freezing winter night. All of these were recorded, posted or written by Americans. Yet all ended up becoming grist for a network of Facebook pages linked to a shadowy Russian company that has carried out propaganda campaigns for the Kremlin, and which is now believed to be at the center of a far-reaching Russian program to influence the 2016 presidential election. A New York Times examination...