At least 15 people are dead and dozens injured in New Orleans after a driver plowed a pickup truck into a crowd of New Year's revelers on Bourbon Street, police and city officials said. The FBI is investigating the incident as an act of terrorism.

Investigators are looking into the attacker's path to radicalization, uncertain how he transformed from military veteran into someone inspired by the Islamic State.

At its height, the group controlled a huge swath of Mideast territory and ruled over millions. Today, its brutal ideology still powers offshoots and inspires "lone wolves."