New York City police announced on Sunday that they had in custody a “person of interest” in the early morning death of a woman who was lit on fire by a man she didn't know on a subway train.

Police apprehended the suspect after receiving a report from three high school students, who had seen images of the suspect taken from surveillance and police body cam video.

“New Yorkers came through again,” said New York City Police Commissioner Jessica Tisch, who described the case as “one of the most depraved crimes one person could possibly commit against another human being.”

Tisch said the suspect and the woman, both of whom have not been identified, were riding a subway train without any interaction between them to the end of the line in Brooklyn at around 7.30am.

After the train came to a stop, surveillance video from the carriage showed the man “calmly” walk up to the victim, who was seated motionless, possibly sleeping, and set her clothing on fire with what appeared to be a lighter.

The woman's clothing then “became fully engulfed in a matter of seconds,” Tisch said.

Officers on a routine patrol discovered the woman on fire, standing in the middle of the carriage. She was declared dead at the scene.

Unbeknownst to the officers, the suspect was seated just outside the train car, on a bench on the platform, Tisch said. Body cameras worn by the officers caught a “very clear, detailed look” at the suspect.

The man had a lighter in his pocket when he was taken into custody, Tisch said. (Reuters)