Chicago police said that the deadly shooting happened Thursday afternoon at the Nadia Fish and Chicken on 75th street and Coles Avenue when an unidentified man walked into the eatery and started firing.
There were a total of four victims; two of whom were brothers.
Dillon Jackson, 20, was found dead outside the restaurant and his 19-year-old brother Raheem was found slumped against a tree near the eatery. Two other victims, Emmanuel Stokes, 28, and Edwin Davis, 32, died inside Nadia Fish and Chicken.
The Chicago Tribune reported that the boys' mother Dana Jackson was working when her sons came in to get food and to visit with her.
According to the outlet, the mother, still wearing her work apron, angrily pounded her fists on the hood of a police car and threatened to kill herself because she was so upset.
"I can't go on, my life is over," Dana said. "I'm about to goddamn kill myself. I was standing right here in the window, they killed 'em right in front of me."
Family members quickly came down to the restaurant to comfort the grieving mother. The boys' grandmother Georgia Jackson, 72, was photographed screaming as a man held her back.
Another photo showed Jackson on the ground crying as family members crowded around her. Jackson told the outlet that the mother called her and told her that Dillon had been shot.
When she arrived to the restaurant, police had discovered Raheem's body not far away. All four victims were pronounced dead at the scene.
In a tweet late Thursday, police spokesman Anthony Guglielmi said that the shooting was believed to be a gang-related retaliation for "another incident." So far, no arrests have been made.