FOREST PARK, Ill. (AP) Authorities in suburban Chicago were reviewing video footage and other evidence Tuesday in their investigation of a shooting aboard a transit train that left four sleeping passengers dead.

The shooting took place before 5:30 a.m. Monday aboard the Chicago areas L system, on a Blue Line train that was moving near where the line ends in Forest Park, a suburb of about 14,000 people thats about 10 miles west of downtown Chicago. A suspect was later arrested on another Chicago Transit Authority L line, according to police.

Forest Park Mayor Rory Hoskins said charges were expected Tuesday.

Three men and one woman were killed, according to the Cook County medical examiners office. Forest Park police said all four were adults, but officials did not yet have exact ages for all of them Tuesday.

Hoskins told several media outlets that the victims were sleeping when the shooting happened.

These victims, likely, never saw it coming, Hoskins told WLS-TV.