The FDNY said probers conducted a "thorough investigation" and found "no conclusive information to arrive at a definitive cause for this fire."
Police have said they have no evidence to support the Transit Authority's theory that a vagrant set the fire to keep warm during the blizzard after a shopping cart was found near the gutted room.
The probe initially focused on a homeless person after two train operators spotted a cart and garbage on fire along the tracks.
Cops have interviewed homeless people over the last week, but have been unable to find the culprit, officials said.
Investigators have ruled out defective wires and sparks from a train as possible causes of the fire.
Transit officials said last week that a five-member task force will be assembled
