US Army veteran Shamsud-Din Jabbar, 42, drove a pick-up truck through a crowd of New Year’s Eve revellers in Bourbon Street, New Orleans, in the early hours of January 1.
The death toll was initially given as 10 but has risen to 15.
People fled screaming for safety into bars and nightclubs as Jabbar mowed down revellers who had been celebrating the New Year.
Dozens of people were hurt during the attack, which is being treated as an act of terror by law enforcement.
Jabbar bypassed safety barriers intended to keep people safe and was “hell bent” on “creating carnage”, according to New Orleans Police Superintendent Anne Kirkpatrick.
She said it wasn’t just an act of terrorism, adding: “This is evil.”
The attacker was killed by police after he got out of the truck and opened fire on officers. Two police who were shot are in a stable condition.
A handgun and AR-style rifle were found inside the truck. A photo circulated among law enforcement officials showed a bearded Jabbar wearing camouflage next to the truck after he was killed.
The intelligence bulletin said he was wearing a ballistic vest and helmet. The flag of the Islamic State group was on the truck’s trailer hitch, according to the FBI.
Detectives found guns and what appeared to be an improvised explosive device inside the truck, along with other devices elsewhere in the city’s French Quarter.