NEW ORLEANS — A US Army veteran driving a pickup truck that bore the flag of the Islamic State group wrought carnage on New Orleans' raucous New Year's celebration, killing 15 people as he steered around a police blockade and slammed into revelers before being shot dead by police.

The FBI said it was investigating the attack early Wednesday as a terrorist act and did not believe the driver acted alone. Investigators found guns and what appeared to be an improvised explosive device in the vehicle, along with other devices elsewhere in the city's famed French Quarter.