CNN
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Four people were killed in a mass shooting on a Chicago commuter train early Monday morning, police said.

“The victims were all passengers on the early morning train,” said Forest Park Deputy Chief Chris Chin. “The train was in motion en route to Forest Park.”

A suspect in the shooting has been arrested. Police believe it was an isolated incident.

“There is no immediate threat and it appears to be an isolated incident on this unfortunate day,” Chin said.

Investigators are working with the Chicago Transit Authority to review surveillance footage of the shooting, which police say occurred in two train cars.

Police said surveillance footage was helpful in identifying the suspect, who was taken into custody about an hour and a half after the shooting. Law enforcement also recovered a firearm.

“While this was an isolated incident, this abhorrent and outrageous act of violence should never have occurred, much less on a public transport train,” the public transport authority said in a statement.

Police have not yet released any information about the victims, but said their next of kin had been notified.

“It’s obviously shocking, you know, it’s rare for a mass shooting to happen,” Chin said. “It’s an isolated incident, so it could happen anywhere. Of course, there’s a little bit more concern because it’s on a mass transit system,” he added. “It’s reasonable for people to feel unsafe.”

According to the Gun Violence Archive, which like CNN defines a mass shooting as one in which four or more people are killed, not including the shooter, there have been 377 mass shootings in the United States this year. In 2024, the U.S. will average 1.5 mass shootings per day.

This is a developing story that will be updated.