Originally Published: 12 JUN 23 00:12 ET

Updated: 12 JUN 23 13:35 ET

By Nouran Salahieh, Celina Tebor and Holly Yan, CNN

(CNN) — [Breaking news update at 1:34 p.m. ET]

The fire that caused part of Interstate 95 to collapse in Philadelphia started after a truck driver took an off-ramp and crashed into a wall, state officials said Monday.

The truck driver was driving northbound on I-95 Sunday morning, took an exit ramp and then lost control, hitting the wall while trying to navigate a curve, Pennsylvania Department of Transportation Secretary Mike Carroll said Monday.

The truck “landed on its side,” and the crash into the wall ignited the fire, Carroll said. The truck was carrying 8,500 gallons of gasoline, according to Pennsylvania State Police.

[Previous story, published at 1:09 p.m. ET]

Massive chunks of the Interstate 95 overpass that collapsed in Philadelphia on Sunday are now being demolished, authorities said Monday.

“Demolition of the collapsed bridges has begun and detours are in place,” the Pennsylvania Department of Transportation said in a release.

“A more exact timeline for the complete rebuilding of the I-95 roadway should be available in the coming days once the engineers complete their review.”

While crews try to clean up the debris, federal investigators are probing the tanker truck fire that led to the collapse – leaving part of the East Coast’s primary highway with major damage that could take months to repair.

Pennsylvania Gov. Josh Shapiro issued a disaster declaration Monday, which allows the state to dip into federal funds and cut red tape to expedite repairs. The roadway is one of the busiest interstates in the region, typically carrying about 160,000 vehicles through Philadelphia daily.

Investigators are monitoring the emergency response as crews sift through rubble to get to the 8,500-gallon-capacity tanker truck – an initial focus of the investigation, said Jennifer Homendy, the head of the National Transportation Safety Board.

It remains unclear whether anyone – including the driver – was trapped in the burning truck. The tanker was carrying gasoline bound for delivery at a local Wawa gas station, Homendy told CNN.

“We have to get in and see what we think happened with the tanker truck,” said Homendy, underscoring the chain of events will remain unclear until investigators can examine the truck’s cab. “There are lots of different scenarios,” she said. Investigators could also consider the structural makeup of the bridge, she added.

Transportation Secretary Pete Buttigieg said Monday that his agency is prepared to help local officials swiftly address the extensive disruption caused by the collapse. “But, just to be clear, swiftly is not going to be overnight,” Buttigieg told reporters at an event hosted by the American Council of Engineering Companies. “We’re talking about major structural work,” he said.

No injuries or fatalities from the highway collapse have been reported. “I found myself thanking the Lord that no motorists who were on I-95 were injured or died,” Shapiro said, describing witnessing “remarkable devastation” during a flyover of the scene.

Northbound lanes collapsed and southbound lanes were damaged due to the intensity of the blaze and were “not structurally sound to carry any traffic,” Shapiro said. Restoring the highway will likely take months, he said, adding that his office was looking into “alternatives to connect the roadway beyond detours.”

Rep. Brendan Boyle, a Democrat whose district includes the collapsed section of the highway, told CNN that “you are literally going to have millions of people in what is one of the largest population centers in the country impacted in a significant way.”

All lanes of I-95 are closed between the Woodhaven and Aramingo exits, the city of Philadelphia said. Some surrounding streets are also closed for the emergency response. Drivers heading southbound can take Route 63 West, US 1 South, I-76 East and I-676 East, the city said. Northbound drivers can detour to I-676 West, I-76 West, US 1 North to Route 63 East.

Leslie Richards, general manager of the Southeastern Pennsylvania Transportation Authority (SEPTA), said the agency is adding extra capacity and service to other transportation routes and evaluating all options in assisting travelers as they work around the highway collapse.

“In order to accommodate travel through the city and region following the I-95 collapse, SEPTA will provide added capacity and Service. on the Trenton, West Trenton and Fox Chase Lines,” SEPTA, the sixth-largest public transportation agency in the US, said in a statement.

The Department of Transportation will offer whatever assistance is possible with repairs and reconstruction, Buttigieg said Sunday.

“Secretary Buttigieg has assured me that there will be absolutely no delay in getting federal funds deployed to quickly help us rebuild this critical artery. I-95, of course, is a critical roadway that supports our economy and plays an important role in folks’ everyday lives,” Shapiro said.

The commercial tanker truck caught fire around 6:20 a.m., causing a section of the overhead northbound I-95 highway to collapse atop the truck, authorities said. The cause of the fire is also under investigation.

By Sunday afternoon, the fire was contained but firefighters remained at the scene as a precaution “because of the large volume of product that was involved,” Philadelphia Fire Department Deputy Commissioner Jeffrey Thompson said.

Crews worked through the night to clear the collapsed section of the road.

Officials warned residents to avoid the area and to expect delays of trash collection and bus routes in the area.

Video shows ‘dip’ in the road before collapse

Mark Fusetti was driving south on I-95 in Philadelphia to pick up his son from the airport on Sunday before the collapse when he saw large plumes of dark smoke and began filming, initially thinking there was a brush fire.

Video he filmed on his cellphone appears to show his car and other vehicles driving over a “dip” along I-95 as smoke billowed from under both sides of the highway. He said he was startled by the dip. “It felt like you drove off a curb,” he said.

“I realized what happened when I looked in my rearview mirror. I see 95 – all of the cars stopping and then I learned, shortly after that the road had just collapsed and what was really going on,” Fusetti told CNN’s Jim Acosta on Sunday.

“It looked like we had a lot of heat and heavy fire underneath the underpass,” said Philadelphia Fire Department Battalion Chief Derek Bowmer.

With thousands of tons of steel and concrete on top of where the fire was burning, firefighters initially faced a challenge getting to the seat of the fire, Bowmer said during a news conference Sunday morning.

There were also explosions around the highway collapse caused by “runoff of maybe some fuel or gas lines that could have been compromised by the accident,” Bowmer

President Joe Biden has been briefed on the collapse, according to a tweet from White House press secretary Karine Jean-Pierre.

A spokesperson for the Federal Highway Administration said administrator Shailen Bhatt would be in Philadelphia on Monday to “offer federal support and assistance.”