4 people were shot dead as they slept on a subway train near Chicago. A suspect is in custody

An elevated train moves along the tracks on August 16, 2024, in Chicago.

Four people who appeared to be sleeping on a Chicago transit train were fatally shot Monday morning, police said.

“The victims, they were all passengers on the train early this morning,” said Forest Park Deputy Chief Chris Chin. “The train was in motion on the way into Forest Park.”

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The victims may have been homeless, Chin told CNN.

“We don’t know for sure what their social status is. Looking at the videos, they were asleep on the train,” Chin said.

A suspect is in custody for the mass shooting, which police believe to be an isolated incident and a random attack.

Investigators are working with the Chicago Transit Authority to review surveillance footage of the shooting, which police said happened in two train cars: three in one car and the fourth in another.

Police said surveillance video was instrumental in identifying the suspect, who was taken into custody about an hour and a half after the shooting.

“The CTA has great cameras. Their surveillance cameras are excellent,” Chin said.

Chin said three agencies — Forest Park Police, Chicago Police and the CTA — worked together to track and capture the suspect

Chin told CNN the suspect likely exited the train at the Harlem stop on CTA’s Blue Line. He eventually got on the Pink Line, where he was apprehended by police. Law enforcement also recovered a firearm.

“I was able to watch some of the videos and it seems like they were able to track, locate and then identify or at least be able to pinpoint … OK, this is who Forest Park is looking for. And Chicago was able to nab him and we were able to swoop in and take him into custody.”

“Although this was an isolated incident, this heinous and egregious act of violence should never have occurred, none the less on a public transit train,” the transit authority said in a statement.

As investigators continue to comb through the evidence, prosecutors will have 48 hours from the time of his arrest to announce any charges against the suspect.

“This obviously is shocking, you know, it is uncommon to have a mass shooting like this,” said Chin. “This is an isolated incident, so it can happen anywhere. Obviously, there’s a little bit more concern because it is on a mass transit system,” he added. “For people to feel unsafe, it’s justified.”

CTA’s Blue Line spans 27 miles between O’Hare International Airport to Forest Park, passing through or near dozens of neighborhoods and downtown Chicago.

Forest Park Mayor Rory Hoskins said during a news conference the police and fire departments respond to the same area of Forest Park “probably more than any other location in our jurisdiction,” appealing to state leadership for additional public safety resources and law enforcement personnel.

He said he has traveled to Springfield in the past to call for more resources to deal with mental health and opioid addiction.

The mayor described this shooting an “outlier,” saying the city hasn’t really experienced this kind of shooting before.

There have been 377 mass shootings in the United States so far this year, according to the Gun Violence Archive, which, like CNN, defines a mass shooting as one with four or more victims, not including the shooter. There have been an average of 1.5 mass shootings per day in the US in 2024.

This story has been updated with additional information.