On September 11, 2001 an unimaginable horror unfolded that devastated the United States and the world. Fifteen years later we are still gripped by terror, but it has transformed. 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 destruction of the World Trade Center on 9/11 to today to find out how radical organizations have grown to make use of modern propaganda and social media tools in order to cultivate an army of self-radicalized killers. 15 Years of Terror goes inside the minds of this new breed of terrorist to define the new techniques and technologies that can be utilized to help law enforcement cope with this elusive threat and to figure out how psychology and technology can be leveraged to end this dreadful cycle of terror.