A man is being investigated for suspected drink-driving after a pick-up truck slammed into a crowd watching a Mardi Gras weekend parade in New Orleans' Mid-City section, injuring 28 people including children and a police officer.
Asked whether terrorism was suspected, police chief Michael Harrison did not say "No", but added that it looked like a case of driving while intoxicated.
"We suspect that that subject was highly intoxicated," he said.
Twenty-one people were taken to hospital after the crash, five of them in a "guarded" condition.
Seven others declined hospital treatment.
The victims' ages ranged from three or four to people in their 30s and 40s, said city emergency services director Dr Jeff Elder.
Mr Harrison said the injured police officer was undergoing tests and in "good spirits".
One woman at the scene told The New Orleans Advocate that a truck whisked by her just feet away.
Carrie Kinsella said: "I felt a rush, it was so fast."
Kourtney McKinnis, 20, told the Advocate that the driver of the truck seemed almost unaware of what he had just done.
"He was just kind of out of it," she said.
The accident came during one of the busiest nights of Mardi Gras when thousands of people throng the streets of Mid-City to watch the elaborate floats and clamour to catch beads and trinkets tossed from riders.
The accident scene was cordoned off with police barricades.