The fire happened at the Girard Station of the Broad Street Line at around 2:00 p.m. on Tuesday. Witnesses said a bag was on fire.
No injuries were reported, but the fire caused an estimated $25,000 in damage. After examining the scene, investigators have ruled that it was intentionally set.
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.
It was a disruption that stranded thousands of commuters during rush hour.
"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.
The investigation is being conducted by fire investigators from ATF, the Philadelphia Fire Marshal's Office, and SEPTA Police Department.
Anyone with information is asked to call 1-888-ATF-FIRE (1-888-283-3473)