Four people were shot dead while sleeping in their seats on a Chicago-area train on Monday.

The incident happened on an early morning commuter train in the suburb of Forest Park, about 10 miles west of downtown Chicago.

Police responded to the scene at approximately 5:30 am and found three victims deceased. The fourth victim later died in the hospital.

The suspect, who fled the scene, was later arrested on another train with a gun in their possession.

Police say the attack appears to be an isolated incident with no further threat to the community. The victims’ identities have not been released, and the motive behind the shooting is still unknown.

The local mayor, Rory Hoskins, described the incident as a “horrible tragedy” that left four people dead on Labor Day weekend.

“Our police department and our fire department respond to this location probably more than any other location in our jurisdiction,” Hoskins said.