The fire broke out around 6 a.m. underneath I-95 near the Cottman Avenue exit in the Tacony section of the city.
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"We're getting multiple calls for a fire," said one emergency dispatcher.
A first responder can be heard saying, "We need this off-ramp shutdown. I have no idea what is on fire."
SEE ALSO: Portion of I-95 to be closed for 'months' after fire, collapse in Philadelphia: Officials
Portion of I-95 to be closed for 'months' after fire, collapse in Philadelphia
Video from the scene showed a massive concrete slab had fallen from I-95 onto the road below. Pennsylvania Governor Josh Shapiro said his flight over the area showed "just remarkable devastation."
"When I realized the flames are coming up on the side of me, and as they hit me, I just like got really scared. And like at the end, I'm just like, "Oh my God. Thank you, God, for getting me through this," said Angela Lewis, of Kensington, who witnessed the fire on the highway.
The northbound lanes of I-95 were gone and the southbound lanes were "compromised" by heat from the fire, said Derek Bowmer, battalion chief of the Philadelphia Fire Department. Runoff from the fire or perhaps broken gas lines caused explosions underground, he added.
Shapiro says the highway near the scene of the collapse could be closed for months as repairs are made.
Investigators are now working to determine what caused the tanker to go up in flames.
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Sources tell Action News the tanker truck was carrying 8,500 gallons of 87-octane fuel
The driver of the tanker is unaccounted for on Sunday night, sources said.
SEE ALSO: 'The road's falling': Philly drivers capture heavy flames before I-95 collapse
'The road's falling': Philly drivers capture heavy flames before I-95 collapse
They say the man is an experienced driver, and that something likely happened as he was coming off the ramp of the highway.
There is no word yet on the driver's identity.
Video from people who were driving on I-95 before the collapse are shocking. One woman took video from her Lyft ride at 6:21 a.m. The road collapsed about one minute later.
Other videos show the road starting to buckle.
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One of those videos was taken by Lisa Taormino of Langhorne, Pa. She said she was on her way to work and, by the time she realized the smoke and flames were on the highway, it was too late.
She was surrounded and her car dipped down into the road.
Taormino can be heard saying, "Oh my God, the road's falling," while traveling past the scene.
Pennsylvania State Police, Philadelphia police, and the Pennsylvania Department of Transportation have established the detour around the I-95 collapse.
SEPTA will also be adding extra capacity will be added to its services following the highway's collapse.
SEE ALSO: Detours posted following I-95 collapse in Philadelphia | What residents, visitors should know