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 area’s 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 that’s about 10 miles (16 kilometers) 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 examiner’s 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.

Police said a preliminary investigation shows the victims were on two different cars as the Blue Line train was headed toward Forest Park. The Blue Line runs 24 hours and stretches from Forest Park through downtown Chicago to O’Hare International Airport. It runs both below and above ground.

CTA said security camera video footage “proved to be vital” in helping investigators.

“Although this matter remains under investigation, all current information points this being an isolated incident,” CTA President Dorval Carter Jr. said in a statement.