Atlanta's notorious traffic is expected to be more tangled than usual after a massive fire caused a bridge on Interstate 85 to collapse, completely shutting down the heavily traveled highway.



Georgia's top transportation official said there's no way to tell when the highway, which carries 250,000 cars a day, can be safely reopened to traffic in either direction following the collapse, which happened Thursday afternoon during rush hour.



"We will have to continue to evaluate the situation and adjust as we do," Department of Transportation Commissioner Russell McMurry said. "This incident — make no bones about it — will have a tremendous impact on travel."



The interstate is a major artery for the U.S. South and a thoroughfare for traffic heading north and south through Atlanta. The bridge collapse effectively "puts a cork in the bottle," Georgia State Patrol Commissioner Mark McDonough said.



Georgia Gov. Nathan Deal declared a state of emergency, saying the state is mobilizing resource to try to keep traffic disruption to a minimum while emergency work continues.



The Metropolitan Atlanta Rapid Transit Authority, or MARTA, planned to increase rail services and have additional staff on hand to help passengers figure out how to get where they're going.



It began with a fire that The Atlanta Journal-Constitution reports burned for more than an hour under I-85 northbound near Piedmont Road, spewing clouds of black smoke skyward. The interstate has been closed indefinitely and the Department of Transportation warned all motorists to stay off I-85.



Traffic was bumper to bumper on nearby surface streets Thursday night as people scrambled to find alternate routes.



However, officials said no one was hurt despite dramatic images of towering flames and plumes of smoke.



"This is about as serious a transportation crisis as we can imagine," Atlanta Mayor Kasim Reed said.



Rose Diggs told The Atlanta Journal-Constitution that she lives less than a mile from the fire site but couldn't get home because of blocked surface streets. She said she was told to walk despite being disabled, "but it's raining and dark."



Capt. Mark Perry of the Georgia State Patrol told the newspaper the agency doesn't know what started the fire beneath the bridge but terrorism is not suspected.



Gov. Deal told reporters that some PVC plastic materials in a vehicle may have caught fire.



"I do not know why they did or what the source of their transport was," Deal said Thursday. "But those are questions that will hopefully be answered at least by tomorrow morning."



Atlanta Fire Department spokesman Sgt. Cortez Stafford said no cars were on the overpass when it fell.



"Our guys got here quickly and shut down the interstate and said, 'No one else is driving over this bridge,'" he said.



Firefighters noticed chunks of concrete falling from the bridge and got out of the way just minutes before it collapsed, Stafford said.



Deal said inspectors were at the scene and they've contacted the original company that built the bridge to come in and assess the extent of the damage.



"We're trying to determine everything we can about how quickly can we repair it and get it back in service," Deal said. "I can assure you we will do everything to expedite the repair and replacement of that section of the bridge."