A Chicago man has been charged with murder in the deaths of four people who were shot on Labor Day while riding a train in the Illinois suburb of Forest Park, police said.

Rhanni S. Davis, 30, faces four counts of first-degree murder in relation to what officials described as execution-style killings of the four passengers as they slept, police announced at a Tuesday news conference. The shooting took place before 5:30 a.m. Monday aboard a Chicago area L train in Forest Park, a suburb about 10 miles west of downtown Chicago.

Forest Park police took Davis into custody Monday on another Chicago Transit Authority line before formally charging and identifying the suspect Tuesday.

4 passengers fatally shot on Labor Day on 2 train cars

The 911 call came in shortly before 5:30 a.m. Monday initially reporting that three people had been shot on a Blue Line train in transit near where the line ends in Forest Park.

The Blue Line train, which operates 24 hours a day, runs from Forest Park through downtown Chicago to Chicago O’Hare International Airport.

Police who responded to the scene located four gunshot victims. Three people died at the scene, while the fourth person was taken to a local hospital and later pronounced dead, police said Monday in a news release.

The preliminary investigation revealed that the train was on the move when the victims were shot at close range on two different cars, according to police.

All four victims appeared to have been sleeping and likely didn't even see the gunman before they were killed, Forest Park Deputy Police Chief Christopher Chin previously told USA TODAY.

"It's believed to be random," Chin said.

Three of the victims were identified as Margaret Miller, 64, Simeon Bihesi, 28, and Adrian Collins, 60, the Cook County Medical Examiner's Office confirmed to USA TODAY. A fourth victim, a male, has not been publicly identified because his family has not yet been notified.

Rhanni Davis due in court Wednesday

After the shooting, the suspect attempted to flee but was identified on video surveillance and taken into custody by 7 a.m. on another L line, police said. A firearm was also recovered.

Davis' suspected motive for shooting was not immediately clear, but Chin said Monday that it was an “isolated incident.”

Davis will appear for a court hearing at noon on Wednesday, the Cook County State's Attorney's Office confirmed to USA TODAY.

In a statement provided Wednesday to USA TODAY, Chicago Transit Authority President Dorval R. Carter called the fatal shooting "the definition of a heinous crime and tragedy."

"We extend our condolences to those who lost their loved ones as a result of this senseless act of crime," Carter said, adding that CTA is assisting law enforcement in the ongoing investigation. "The safety and security of our riders and employees is and remains our number one priority."

In a Monday news briefing, Forest Park Mayor Rory Hoskins said the slayings left the community shaken and rattled. Police, Hoskins said, are used to calls to the area due to the busy transit line, but he noted that fatal shootings rarely occur there.

“It’s a horrible tragedy that four people are dead on Labor Day weekend,” Hoskins said Monday.

Hoskins did not immediately respond Wednesday to USA TODAY's request for comment.