Police released an image of a man whom they were seeking.
Investigators now say they believe the arsonist came across a homeless man sleeping next to a cart on the number 2 train, and lit the cart on fire last Friday.
The homeless man is in critical condition.
The subway motorman, 36-year-old Garrett Goble, was killed.
Another 12 people were treated for minor injuries, including five firefighters.
Police are still searching for the suspect they believe set the fire.
"There are other fires that we are looking at, we have to see if they are connected to this, 86th street and 96th street, within the transit system," said NYPD Deputy Chief Brian McGee. "We're also looking at a fire on 116th Street as we speak. The most important thing I need is witnesses to come forward, 800-577-TIPS."
The MTA has issued a $50,000 reward for information that leads to an arrest.
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