Georgia governor Nathan Deal said PVC piping stored under the highway may have fuelled the blaze.
A huge fire broke out on an elevated highway in the US city of Atlanta on Thursday evening, causing a large section of it to collapse.
Flames leapt several storeys high above the Interstate 85 highway before the collapse at 19:30 local time (23:30 GMT), reports said.
Residents said they thought the sun had set early or a storm had arrived because of the thick black smoke.
There were no immediate reports of casualties in the incident.
But the fire caused widespread traffic jams and road closures, stranding many motorists for hours.
Georgia Governor Nathan Deal declared a state of emergency in Fulton County, which covers most of the Atlanta area.
He said the fire appeared to have been fuelled by a large pile of PVC piping under the structure, although it was unclear why it was stored there.
Airport firefighters were called in to spray foam typically used in airline crashes onto the flames.
Atlanta police told residents not to drive in order to keep roads clear for emergency services.
Government offices in Atlanta were set to open late on Friday to give people extra time to travel to work.
