Investigators believe it was an isolated and random attack, police said Monday.
CTA workers discovered the horrific scene around 5:30 a.m. just as the inbound Blue Line train arrived at the station. Forest Park police said they received a 911 call that three people were shot on a train.
Officers arrived on the scene and found four shooting victims. Three were pronounced dead on the scene. A fourth, who was found in a different train car, was transported to Loyola University Medical Center, where they later died.
"Shocking, you know, it is uncommon to have a mass shooting like this, especially this close to home," said Forest Park Police Deputy Chief Christopher Chin.
Forest Park Mayor Rory Hoskins said the shooting is believed to have happened one stop away, near the Harlem Blue Line station. Hoskins said the victims, all adults, were sleeping when they were killed.
"These victims, likely, never saw it coming," Hoskins said. "They were executed on Labor Day. In our community, people go to the pool. They go to the park. You know, they barbecue. But, today, a lot of people were calling the mayor's office, expressing concern and asking if they were safe."
They 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.
Police said the shooter fled the scene. But with the help of CTA surveillance video, officers within 90 minutes responded to a CTA Pink Line station, where they took a suspect into custody and recovered the gun allegedly used in the shooting.
"We believe he got off at the Forest Park stop, and he was apprehended at a Pink Line station in Chicago somewhere, so he may have gotten on a train going the opposite direction," Hoskins said.
Police said the shooting happened on two different train cars.
"I mean, it's a horrible situation," Chin said. "It is definitely something you don't want to wake up to. It's a Monday morning on a holiday. Everyone is supposed to enjoying their time off, time with their families, it's Labor Day. To actually have to be here to address a situation like this, it's horrible."
Forest Park police and the West Suburban Major Crimes Task Force are investigating the shooting.
Hoskins said law enforcement responses to the station have increased for things like overdoses.
"Over the last two years, our number of responses have increased exponentially," Hoskins said.
Hoskins said Forest Park is the only suburban community to have two major train lines end in it, and because of that, they have seen increased criminal activity at the station.
"We're calling on our leadership, countywide, statewide, to invest more resources to support a small community like Forest Park," Hoskins said.
Forest Park police provide update on CTA shooting
Hoskins said those resources can be a combination of enhanced security personnel and putting more workers on trains.
"For people to feel unsafe, you know, it's justified," Chin said.
The suspect and victims involved in this shooting have yet to be identified.
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 CTA has a new zero-eyes security system, which is 24/7 surveillance on CTA train platforms that can detect guns. However, that technology is only at select stations, and the CTA said it has not been a factor in the Forest Park investigation.
The CTA said in a statement, "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.
"As soon as this matter was reported, CTA immediately deployed resources to assist the Forest Park Police in their investigation into the matter, including review of all possible security camera footage, which proved to be vital in aiding local enforcement.
"We commend both the Forest Park Police Department for their thorough and collaborative efforts to provide information to partner agencies; as well as the Chicago Police Department whose quick actions led to the apprehension of a suspect in this matter.
"CTA will continue to work with local law enforcement as part of this ongoing investigation."
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.
WLS and CNN contributed to this report.