Sources say detectives are looking into the possibility that the person in custody is linked to a fire that broke out at City Hall on Friday.
Surveillance video from both incidents show a general similarity in appearance that has prompted detectives to obtain better video to see if it is, in fact, the same person.
ABC7 is waiting to hear more details from investigators over this person of interest as a 26-year-old woman, in critical condition, is being treated for third-degree burns.
CPD questions person of interest after woman set on fire on CTA train
As the peak of rush hour came to an end on Tuesday, CTA riders hoped Chicago police have the person responsible for the incident.
"That's an absolutely horrible thing to have happened on the L. But I have to say, the past three or four years, the L has not been a pleasant experience, and it's a dangerous experience, at worst," said CTA rider Tarick Loutf.
Chicago police said a 45-year-old man got into an argument with a 26-year-old woman around 9:25 p.m. Monday.
That fight turned violent when, investigators say, the man poured an unknown type of liquid on her and then set her on fire with a lighter.
"I just saw a lady who was lying on the ground, and they were trying to give her CPR. Looked like she was in an awful lot of pain," said witness John K.
The man ran away after the train stopped at Clark and Lake.
The injured woman, witnesses say, fell to the ground after she was able to get off the train.
"Just seen her lying on the ground, hyperventilating, and in a lot of pain; she was in really bad shape," John K. said.
The Chicago Transit Authority said in a statement, "The CTA has been working closely with the Chicago Police Department and the Public Transportation Unit detectives embedded at the Strategic Decision Support Center dedicated to CTA to support their investigation, which has now led to the arrest of a person of interest."
The 26-year-old was taken to Stroger Hospital in critical condition with severe burns.
"It's very dangerous. I have gotten robbed like three times before. But I don't have another way of transportation," said CTA rider Jessica Perez.
Detectives have a person of interest in custody as investigators continue to review surveillance video in the area.
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