MAYWOOD, Ill. (AP) — A 30-year-old man charged with killing four people while they slept aboard a Chicago-area transit train was expected in bond court Wednesday.

The Labor Day shooting happened before 5:30 a.m. aboard Chicago’s L system, on train that was moving near where Blue Line ends in suburban Forest Park, about 10 miles (16 kilometers) west of downtown Chicago. Rhanni S. Davis, of Chicago, was arrested later Monday on another Chicago Transit Authority train line and charged Tuesday with four counts of first-degree murder.

Investigators haven’t determined a motive.

“The question of why may never be answered,” Cook County State’s Attorney Kim Foxx said at a news conference Tuesday.

She called the shootings a “horrific, heinous and inexplicable act of violence” and said more details would come out during Wednesday’s court hearing.

Forest Park Mayor Rory Hoskins told The Associated Press that the victims likely didn’t even see the shooter, saying they were shot at close range as they slept.

Margaret Miller, 64, and three men including Simeon Bihesi, 28, and Adrian Collins, 60, were fatally shot, according to the Cook County medical examiner’s office. All of their addresses were listed as unknown. The fourth person’s name has not yet been released because police were still working to notify relatives.

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A preliminary police investigation found that 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 that suburb through downtown Chicago to O’Hare International Airport. It runs both below and above ground.

Police later found Davis using video footage, Hoskins said.

“This was a very random, isolated incident,” CTA President Dorval Carter Jr. said Tuesday evening.

Public records did not have a listed phone number for Davis. Messages sent to a listed email were not returned. Forest Park police and the Cook County state’s attorney’s office did not respond to messages about Davis’ legal representation. The Cook County public defender’s office didn’t immediately respond to a message Wednesday.

Davis was scheduled to appear in court at noon in suburban Maywood.