On September 11, 2001, an unimaginable horror unfolded that devastated a nation and the world. Fifteen years later, we are still gripped by terror, but it has transformed. The attacks have been coming fast and furious–to Boston, Paris, Brussels, San Bernardino, Orlando, Nice–but they are no longer commanded by a central entity. This is terrorism in the age of the Internet: crowd-sourced violence. In this special report, NOVA traces the evolution of terror strategies from the World Trade Center to today.