A massive fire caused a bridge to collapse Thursday night on Interstate 85 in Atlanta, one of the busiest stretches of roadway in America, backing up rush hour traffic for several miles and creating what the mayor called a long-term "transportation crisis."

The huge fire was reported at 6:21 p.m. ET underneath the bridge on the northbound side — near where the interstate merges with another major artery, State Highway 400 — forcing authorities to close the interstate and turn drivers around during the evening rush.

Then, at about 7 p.m., the bridge collapsed in a flaming heap that spewed thick, black smoke high into the air, where it was visible for miles around Atlanta.

Authorities managed to turn back traffic before the bridge collapsed, and Georgia Gov. Nathan Deal said no injuries were reported. But thousands of motorists were locked in place on the interstate as state troopers worked on a way to get vehicles off the roadway.

Deal declared a state of emergency for Atlanta and Fulton County late Thursday night.

IMAGE: Atlanta freeway fire

Flames billow Thursday from beneath an overpass on Interstate 85 in Atlanta. NBC News Channel

The fire itself was declared under control about 8 p.m., but the collapsed bridge was the principal problem. All lanes of I-85 in the area are expected to be blocked indefinitely, the state Transportation Department said.

"I think it's as serious a transportation crisis as we could have," Mayor Kasim Reed said at a news conference late Thursday. He said that FBI agents were on the scene "to work out what happened" but that the agency had reported no suspected link to terrorism.

Russell McMurry, commissioner of the Georgia Department of Transportation, also described the situation as a "crisis."

At a news conference at 11 p.m. ET — almost five hours after the fire started — transportation crews still couldn't gain access to the scene because of the heat still being generated by the flames, McMurry said.

A department spokeswoman said it was unlikely that inspections of the affected structures could even begin until Friday afternoon at the earliest.

Image: Atlanta bridge collapse

Firefighters survey an overpass that collapsed from a large fire Thursday on Interstate 85 in Atlanta. Georgia Gov. Nathan Deal declared a state of emergency for Fulton County. David Goldman / AP

Atlanta Fire Chief Kelvin Cochran disclosed Thursday night that other parts of I-85 had been "compromised" — including the important interchange over Piedmont Road, which leads to the state Capitol.

In an interview with NBC affiliate WXIA, Cochran credited state troopers with saving lives by managing to stop all of the traffic in the 40 minutes before the bridge collapsed.

"We had heavy fire. We had heavy smoke. We couldn't see through that smoke just like everybody else," he said.

The cause of the fire remained undetermined Thursday night, but Deal said in a televised statement that authorities were focusing on the possibility that polyvinyl chloride (PVC) piping commonly used in construction may have ignited for some reason. WXIA reported that large amounts of PVC pipe were stored under the bridge.

The station had a helicopter at the scene to record the raging fire.

The location is near the ritzy downtown Atlanta district of Buckhead, sometimes called "the Beverly Hills of the South," which is home to some of the city's biggest businesses, medical centers and diplomatic missions.

State and local officials said more than 225,000 vehicles traveled trough the affected area on an average weekday, and they pleaded with Atlantans to do whatever they could to stay away for as long as it takes.

"Most of us are used to our routines. We're used to that one way we go to work and that one way we do our business," said Col. Mark McDonough, chief executive of the state Department of Public Safety. "So this is now the time to change that."

John Eaves, chairman of the Fulton County Board of Commissioners, asked businesses in the area to make plans for flexible work schedules and to encourage employees to use public transit in the coming "days and weeks."

The Metropolitan Atlanta Rapid Transit Authority said it was arranging extended service to accommodate the extra riders.

The DeKalb County School System said all schools would be closed Friday. The Atlanta school system said it was working on a plan to find alternative bus routes through the area.

Atlantans already suffered through the fourth-worst traffic in America and the ninth worst in the world, Inrix, a global transportation research firm, reported last year. But the impact of the collapse is likely to be felt for some time across the Southeast.

I-85 is one of the busiest freeways in the eastern half of the country, carrying vehicles almost 700 miles from southern Alabama through Virginia. It is an important commercial throughway for trucks making deliveries to businesses and warehouses not only in Atlanta but also in other major commercial centers like Montgomery, Alabama, and Charlotte, North Carolina.

WXIA reported that transportation officials were meeting with officials from Deal's office to assess both the immediate and the long-term impacts on transportation and commerce.