A woman died after being set on fire on a New York subway train on Sunday, with a man arrested on suspicion of carrying out the "depraved" attack.
Transit police apprehended the male suspect after receiving a report from three high school students who had recognised the man from photographs and police body cam video that was widely distributed by police and the media.
New York City Police Commissioner Jessica Tisch praised the people in her city for helping authorities apprehend the person who may be responsible.
"New Yorkers came through again," she said, before describing the attack as "one of the most depraved crimes one person could possibly commit against another human being".
Ms 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 local time.
After the train came to a stop, surveillance video from the subway car 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", Ms Tisch said.
The NYPD believe the two were not known to each other.
Officers on a routine patrol at the Coney Island-Stillwell Avenue subway station smelled and saw smoke and discovered the woman on fire, standing in the middle of the subway car.
After the fire was extinguished, emergency medical personnel declared the woman dead at the scene.
Unbeknownst to the officers, the suspect had remained at the scene and was seated on a bench on the subway platform, just outside the train car, Ms Tisch said.
Body cameras worn by the officers caught a "very clear, detailed look" at the suspect and those images were publicly disseminated.
After receiving a 911 call from the teenagers, other transit officers identified the man on another subway train and radioed ahead to the next station, where more officers kept the train doors closed.
Transit police then searched each car and ultimately apprehended him without incident, said chief of transit Joseph Gulotta.
Ms Tisch said the man had a lighter in his pocket when he was taken into custody.
Mr Gulotta said the investigation was continuing, including whether the woman was homeless and the background of the suspect.