WITH US TONIGHT. THOSE FOUR VICTIMS WERE SLEEPING ON THIS TRAIN WHEN THEY WERE SHOT. LIKELY BEFORE IT EVEN ARRIVED HERE AT THIS TRAIN STATION. CTA WORKERS MAKING THE GRUESOME DISCOVERY. TRAIN SERVICES ARE SLOWLY PICKING UP HERE AT THE BLUE LINE STATION IN FOREST PARK. AFTER FOUR PEOPLE, ALL ADULTS, WERE FOUND SHOT. 3 IN 1 TRAIN CAR AND ONE WAS FOUND IN ANOTHER. ALL OF THEM SAID TO HAVE BEEN SLEEPING AT THE TIME OF THEIR MURDERS. THESE VICTIMS, UH, LIKELY NEVER SAW IT COMING. >> THEY WERE EXECUTED. UM ON LABOR DAY. RIGHT. SO LABOR DAY IS GENERALLY A FESTIVE KIND OF DAY. UH, 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. >> THE HORRIFIC SCENE WAS DISCOVERED BY CTA WORKERS AROUND 530 MONDAY MORNING, JUST AS THE INBOUND TRAIN ARRIVED AT THIS FINAL STOP, THE MAYOR SAYS THE SHOOTING IS BELIEVED TO HAVE HAPPENED ONE STOP AWAY NEAR THE HARLEM BLUE LINE STATION WITHIN 90 MINUTES. POLICE SAY THE SUSPECT WAS FOUND AND ARRESTED, STILL WITH A GUN ALLEGEDLY USED IN THE CRIME. >> WE BELIEVE HE GOT OFF AT THE FOREST PARK STOP. HE WAS APPREHENDED AT A PINK LINE STATION IN CHICAGO SOMEWHERE, SO HE MAY HAVE GOTTEN ON A TRAIN GOING THE OPPOSITE DIRECTION. IN A STATEMENT TO ABC SEVEN, A CTA SPOKESPERSON CALLING THIS AN ISOLATED INCIDENT, SAYING, QUOTE, THIS HEINOUS AND EGREGIOUS ACT OF VIOLENCE SHOULD NEVER HAVE OCCURRED. >> NONETHELESS, ON A PUBLIC TRANSIT TRAIN. NOW, THE VICTIMS AND SUSPECT INVOLVED IN THE SHOOTING HAVE YET TO BE IDENTIFIED