By Jason Hanna, Faith Karimi and Steve Almasy, CNN

Drivers in one of the world’s most congested cities woke up to a jarring new reality Friday as they were forced to game out how to get around a collapsed portion of Atlanta’s Interstate 85 — one of the Southeast’s major north-south arteries.

A mysterious fire collapsed an I-85 northbound overpass Thursday evening — injuring no one — and also damaged the southbound portion, forcing the closure of all five lanes in each direction for the foreseeable future.

The shutdown likely sets the city up for traffic nightmares for weeks to come after creating navigation hell on Thursday with jams that extended five miles or more and stranded motorists in the immediate area for hours.

The closure comes at a sensitive time for a city accustomed to gridlock — with hordes of spring break vacationers poised to drive through the regional hub and the Atlanta Braves set to play a preseason game Friday night in their new stadium in the city’s northwest corner.

“I think it’s as serious a transportation crisis as we could have,” Atlanta Mayor Kasim Reed said Thursday evening.

The fire started Thursday evening under an I-85 overpass in north-central Atlanta, north of the highway’s split with I-75.

Officials said they didn’t immediately know what sparked the flames, which morphed into a massive fireball. At first, I-85 motorists drove through the smoke, and firefighters fought the flames below.

“There was a 40-feet or higher wall of fire. Power lines were falling and arcing heavily and falling in the streets,” Cortez Stafford, a spokesman for the Atlanta Fire Department, told CNN.

The elevated span of highway collapsed about 7 p.m. ET, as crews battling the fire got out of danger’s way.

As concrete began falling from under the bridge, firefighters were asked to step back, Stafford said. “Not even two minutes later, the highway fell with a big ‘kaboom.’ (It) knocked our guys back.”

While the highway is normally jammed with cars around that time, there were no fatalities, Reed said.

More than 220,000 cars per day are estimated to drive through that stretch of the interstate. Officials scrambled to come up with alternate routes and encouraged riders to use public transit.

“This is the time to start planning and looking for an alternate route on how you do your business,” Georgia Department of Transportation Commissioner Russell McMurry said Thursday evening.

Social media users posted surreal images showing motorists — before the collapse — choosing to drive into the black smoke that billowed onto the highway as the fire burned beneath them.

CNN’s Eliott C. McLaughlin was driving north on I-85 during the evening rush hour when he saw smoke rising from underneath the elevated highway.

Many cars on the left side of the five-lane section barreled through the thick black smoke. They disappeared into the darkness as they drove, he said.

McLaughlin slowly followed the taillights of an SUV through the smoke.

Soon, interstate traffic was stopped and turned around, creating long jams.

As word of the incident spread, so did the speculation.

Reed, Atlanta’s mayor, did not provide details on what caused the collapse, but said it is not linked to terrorism and an investigation is underway.

“We’ve spoken to the special agent in charge of FBI, there is no evidence this is related to terrorism,” he said Thursday night.

Gov. Nathan Deal said he heard speculation it was caused by some “PVC products that caught fire.”

Two fire trucks from Hartsfield-Jackson Atlanta International Airport in south Atlanta rushed to the scene and sprayed foam on the fallen section and flames.

Authorities worked through the night to access the bridge and ensure the risks from the collapse are contained.

“There is a possibility you could have a further collapse,” Reed said. “We still have personnel at the sight making sure there are no hotspots. We won’t know the area of vulnerability for the bridge until tomorrow.”

The company that originally built the elevated highway would assess the damage, Deal told reporters.

“It’s going to take some time to get it repaired and to get it back in service,” he said, without offering a timeframe for reopening.

MARTA, Atlanta’s rail and bus system, will offer extended service through the weekend.

One school district, in nearby DeKalb County, canceled classes for Friday. Schools in Atlanta will be open, Reed said, and city and state offices won’t open until 10 a.m.

Most structural materials lose strength when subjected to high temperature, meaning the concrete could have been compromised by the heat, said Reginald DesRoches, a professor at Georgia Tech.

It is too early to say how long it will take to repair the highway, he said.

“It certainly can take anywhere from several weeks to several months,” he said. “The surrounding sections of the highway will be evaluated to determine if any damage was sustained from the heat. It is probably prudent to check both sides of the adjacent sections (northbound and southbound).”