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. How have radical organizations grown to make use of modern propaganda and social media tools in order to cultivate an army of self-radicalized killers? What is going on inside the minds of this new breed of terrorist? What new techniques and technologies can help law enforcement cope with this elusive threat? And how can psychology and technology be leveraged to end this dreadful cycle of terror?