A woman is in critical condition after she was set on fire riding a Chicago Transit Authority train, police said.

Police said a person of interest was taken into custody in connection with the incident Tuesday. No details about the person were released.

The woman, 26, was riding what Chicagoans call the “L” on Monday night when she got into an argument with an approximately 45-year-old man, police said in a statement.

The verbal altercation turned physical when the man poured a liquid on the woman and ignited it, causing her to catch on fire, police said. It is not clear what the liquid was.

When the train stopped, the attacker fled and the woman got off the train and fell to the ground, police said.

The fire was put out before police got to the scene.

The woman sustained severe burns all over her body and was taken to Stroger Hospital in critical condition, the police statement said.

An investigation continues.

According to NBC Chicago, passengers saw paramedics load the burned woman into an ambulance. A witness told the station that the woman appeared to be in a "really bad state."

"I just saw a lady that was laying in the ground, and they were trying to give her CPR," the witness told NBC Chicago. "And she looked like she was in an awful lot of pain."

The train lines were shut down for hours, NBC Chicago reported, and service resumed around 1:30 a.m. Trains were running on schedule Tuesday morning.

Rebecca Cohen is a breaking news reporter for NBC News Digital.