It began when the Philadelphia Fire Department responded to the report of a fire at the Broad and Girard subway station at 1:50 p.m.
An Action News viewer shot video (seen in the above video) of the chaos that followed the smoky fire on the subway platform, spewing thick black smoke unto the sidewalks.
"There was a bag on fire, but wasn't nothing happening. 5 minutes later - boom, boom!" a witness said on the video.
It was a disruption that stranded thousands of commuters during rush hour.
Witnesses had no trouble recalling what they saw and heard.
"You saw a bag and the bag was exploding, it was just a regular bag and it started exploding, it was booming and all over," witness Dan Hosmi of Northeast Philadelphia said.
"Somebody somehow set a bag on fire, they had a big black bag and it lit up, I don't know what was in there, but it started a major fire," witness Peter Muse of North Philadelphia said.
Heavy smoke could be seen coming from the vents on street level.
"We were getting ready to go down on the platform and all of a sudden the whole ground shook and a bunch of smoke started pouring out of the vents on the east side of Broad Street," witness Robert Anglim of North Philadelphia said.
The fire department evacuated the subway station and trains were stopped in both directions.
Some passengers say the fire temporary left them stranded on the train.
Elisha Cooper was one of them; she was with her 3 young children.
"It was people panicking, having anxiety attacks, it was people crying and different things like that, like my son for example, he was crying and I was afraid that he was going to pass out while we were on the train," Cooper said.
Seth Sykes of North Philadelphia was another stranded passenger.
"They wouldn't let us off for at least a half hour and the smoke kept coming into the train, so finally people started becoming irate, they let us out eventually, but we coughed all the way to get out of the station," Sykes said.
The fire was placed under control by 2:15 p.m. No injuries were reported.
Investigators say it's too early to say how the fire started but it appears someone set a backpack on fire and threw it into a shed used to store construction debris.
Service has been restored except for the Broad-Ridge Spur due to water damage. The Broad Street Line trains are still not able to stop at the Northbound Girard Avenue station due to problems caused by the fire.