A Chicago man has been charged with murder in connection with the fatal shootings of four passengers who appeared to be sleeping on a Chicago Transit Authority train Monday morning, police say.

Rhanni S. Davis, 30, was charged Tuesday with four counts of first-degree murder, the Forest Park Police Department said in a news release. A news conference will be held later Tuesday with more details, police said.

Davis was detained about 90 minutes after the shooting was reported around 5:30 a.m. on the Blue Line train heading to the Forest Park suburb west of downtown.

Police recovered a firearm and said Monday they believe this was an isolated and random attack.

Authorities are now working with the Chicago Transit Authority to investigate the shooting, which police said happened in two train cars: three victims in one car and the fourth in another.

Three victims were pronounced dead on scene and the fourth was taken to a hospital in Maywood, where they were later pronounced dead, police said in a news release.

Two of the four passengers were identified as 60-year-old Adrian Collins and 28-year-old Simeon Bihesi, the Cook County Medical Examiner’s office told CNN on Tuesday. The two other victims were described as a man and a woman, but no additional identifying information was available.

Surveillance video was instrumental in identifying the suspect, police said. As investigators continue to comb through the evidence, prosecutors have 48 hours from the time of the suspect’s arrest to announce any charges.

CTA’s Blue Line spans 27 miles between O’Hare International Airport and Forest Park, passing through or near dozens of neighborhoods and downtown Chicago. Service on the line resumed Monday evening.

The victims in the Labor Day shooting may have been homeless, Forest Park Deputy Chief Chris Chin told CNN.

“We don’t know for sure what (the victims’) social status is. Looking at the videos, they were asleep on the train,” Chin said.

“Although this was an isolated incident, this heinous and egregious act of violence should never have occurred, none the less on a public transit train,” the transit authority said in a statement.

“This obviously is shocking, you know, it is uncommon to have a mass shooting like this,” said Chin. “This is an isolated incident, so it can happen anywhere. Obviously, there’s a little bit more concern because it is on a mass transit system,” he added. “For people to feel unsafe, it’s justified.”

Forest Park Mayor Rory Hoskins said the shooting is an “outlier” for the city.

Hours after the shooting, another passenger was seriously injured in a stabbing on the Red Line, Chicago police said in a statement. The incident began as a verbal altercation between a 37-year-old victim and an unknown man, who at some point took out a “sharp object” and repeatedly stabbed the victim, police said.

“The victim fled from the train at the Wilson Red Line station, and relocated, on foot, to the 4700 block of N. Broadway. The victim was then transported to Illinois Masonic Hospital initially listed in critical condition,” police told CNN.

A suspect in the stabbing has yet to be found and detectives are investigating.