An Oak Lawn deputy fire chief who responded to the fire on his way home from work witnessed the suicide, the Chicago Tribune reported. Fire officials said he heard noises from the garage and opened the door, finding a man sitting in a vehicle inside.
In the house, firefighters found the bodies of another man, a 68-year-old woman and a 5-year-old boy. The identities of the victims had not been released late Wednesday.
The adults were all shot, investigators said. The cause of the boy's death was unclear.
Neighbors told WMAQ-TV that the house was occupied by a couple in their 60s who had lived in Oak Lawn for many years. They said a 36-year-old son recently moved in with them and their 5-year-old grandson had been spending the night there after a birthday party Tuesday.
"They really were fantastic people, fantastic neighbors, do anything for us, we'd help them out and stuff. They're going to be really missed around the neighborhood," Tom Repenty said. "The son was great, too. We'd always see him outside, mowing the lawn, we'd chit-chat. He was a mathematician, really smart kid, so it's just a tragedy."