Four people were killed in a shooting early Monday morning on a Blue Line train in Chicago.

The shooting occurred around 5 a.m. local time in Forest Park, Illinois, ten miles outside of Chicago, ABC 7 reported. Three people were pronounced dead at the scene, while the fourth victim was taken to a nearby hospital where he later died.

The victims, whose identities have not been publicly released, were likely asleep at the time of the shooting and may have been homeless, the Forest Park Police Department told CNN.

“We don’t know for sure what their social status is. Judging by the footage, they were sleeping on the train,” Forest Park Deputy Chief Chris Chin told the station.

According to ABC 7, police have arrested a suspect in the shooting on Chicago’s Pink Line train after he fled the scene. Officers seized the weapon during the arrest.

“While this was an isolated incident, this abhorrent and egregious act of violence should never have occurred, much less on a public transit train,” the Chicago Transit Authority said in a statement.

“Immediately after this matter was reported, CTA immediately dispatched resources to assist the Forest Park Police Department in their investigation of this matter, including reviewing all available security camera footage, which proved critical in assisting local law enforcement,” the statement continued.

The investigation has been turned over to Forest Park police and the West Suburban Major Crimes Task Force. Officials say it was an isolated incident and there is no longer a threat to motorcyclists, according to ABC 7.

“We commend both the Forest Park Police Department for their thorough and collaborative efforts to provide information to partner agencies, and the Chicago Police Department whose quick actions led to the apprehension of the suspect in this case,” the CTA said in a statement.

Forest Park Mayor Rory Hoskins called the shooting an “exception” in a statement Monday.

“To the extent that our leaders in Springfield are considering any kind of reorganization of CTA mass transit, we would ask them to consider investing additional resources in terms of security, public safety, law enforcement officers, etc., to support a small community like Forest Park that has to respond to two major rail lines terminating in Forest Park,” Hoskins said.

The incident occurred a week after the CTA implemented a new artificial intelligence program that can automatically detect when a motorcyclist is drawing a weapon, “Chicago Tribune” reports.