Two people -- a 17-year-old boy and a 39-year-old woman -- were injured and taken to area hospitals, treated and released. The shooting occurred around 9:30 p.m. at The Connect store in the 500-block of East 130th Street. It sells groceries and cell phones.
One of those who died may be a suspect in the shooting. No motive is yet known.
The four people shot dead are all males: a 17-year-old, and three men ages 25, 30 and 46. Brandon Phillips, who works at The Connect, told ABC7 that he believes the four men killed may have all been his co-workers.
Phillips and customers also say that one of the victims was an armed security guard whose white Cadillac they say has been parked in front of the crime scene since Wednesday night.
Dozens of people were outside the store grieving Thursday morning.
Police say several guns were involved, and several people are in custody. They are all being held at Area 2.
There was no word on when the store will reopen.
The three men were all pronounced dead at the scene, police News Affairs Officer Hector Alfaro said. The teen later died at Roseland Community Hospital, he said.
The Cook County Medical Examiner's office confirmed the fatalities but was not releasing their identities early Thursday. The names of the victims are expected to be released Thursday afternoon.
The Sun-Times is reporting that the teen who died went to the hospital without an ambulance.
>A Chicago Fire Department spokesman said paramedics responded to the scene, and confirmed they did not take anybody to area hospitals.
Calumet Area detectives remained on the scene early Thursday.
The Sun-Times Media Wire contributed to this report.