Once dependent on leaflets and videotapes, terrorist groups now use social media as a chief recruiting tool. Each day, Twitter users tweet an average of 500 million times and more than a billion people log into Facebook. Instead of relying on humans to finger dangerous content, a computer-science professor at Dartmouth is proposing a system that proactively flags extremist photos, videos, and audio clips as they’re being posted online. He teamed up with the Counter Extremism Project, a nonprofit led by a star-studded roster of former government officials, to propose a program that would help online platforms keep extremist content off the internet. Modeled off the child-porn detection system, the new program would establish a central clearinghouse that would maintain a database of extremist content and distribute unique fingerprints of each photo, video, and audio file to the platforms that want to filter for this content. The clearinghouse would be called the National Office for Reporting Extremism, or NOREX.