Sources said the victims were father and daughter, but investigators have not confirmed that relationship. The Cook County Medical Examiner's office has not released the victims' identities.
Emergency crews responded around 9 p.m. to a report of a fire in the 1600-block of South California Avenue in the city's Douglas Park neighborhood. That's when they discovered the two bodies.
Police said they are investigating the deaths as a "domestic-related murder-suicide."
Illinois Fire Marshal vehicles were on the scene early Friday morning. Investigators were seen pulling bags of evidence out of the building. Neighbors said the fire appears to have started on the first floor.
Kat Belmares, who lived above the victims, stopped to remember them as she moved out of her fire-damaged apartment.
"She was a sweet little girl. A very sweet little girl," Belmares said.
Neighbors said they have been struggling to make sense of what happened and wanted to send their thoughts and prayers to the victims' relatives.
"It's been hard. We don't know quite what happened downstairs. I'm not really sure what's going on. I feel saddened for their families," Belmares said. "It's all kind of setting in. This is a lot of shock right now. We've cried a lot. I think all of us have. I can't imagine what they're going through."
The cause of the fire remains under investigation.