Four people were shot to death on a Chicago-area train on Monday morning, according to Illinois authorities.

Authorities were called to a train station in Forest Park, a village in Cook County, at around 5:30 a.m. on Monday. The shooting occurred on a Chicago Transit Authority (CTA) Blue Line train.

Three people were pronounced dead on the scene, while the fourth victim died at a local hospital. Forest Park Police Department deputy chief Chris Chin told reporters that no additional information about the victims is known at the time.

According to FOX 32 Chicago, a suspect fled the scene but was eventually arrested by authorities. The suspect was reportedly found on a CTA Pink Line train.

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"A weapon was recovered," Chin said. "There is no immediate threat. This appears to be an isolated incident on this unfortunate day."

Forest Park Mayor Rory Hoskins said that the Forest Park train station has been the subject of an inordinate amount of emergency calls.

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"It’s a horrible tragedy that four people are dead on Labor Day weekend," Hoskins said. "Our police department and our fire department respond to this location probably more than any other location in our jurisdiction."

In a statement, the CTA called the shooting "heinous and egregious," and promised to assist law enforcement with their investigation.

"This heinous and egregious act of violence should never have occurred, nonetheless on a public transit train," the organization said.

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Authorities are actively investigating the incident. No additional information is known at this time.

The Associated Press contributed to this report.