Authorities initially did not know the victim had been murdered because of the condition of his body.
Firefighters were called around 9:15 p.m. Monday to the three-story apartment building in the 1800-block of 51st Avenue in Cicero after reports of an apparent fuel explosion in the middle-floor unit.
"I would never expect something like this to happen. There's only like so much that happens here, maybe someone having a verbal confrontation, but nothing this extreme," said Alex Giron, who lives nearby.
Giron was among the neighbors who heard the apparent explosion just after 9 p.m.
Officials say after the fire was extinguished, a middle-aged man was found dead inside that apartment, and initially, investigators did not suspect foul play.
"Then, we see around midnight a body being wheeled off in the gurney," Giron said.
But later, the Cook County Medical Examiner's Office discovered that the man had been shot in the head and ruled his death a homicide.
Cicero officials say because of the condition of the body, they did not initially see that he had been shot. But now, police are conducting a criminal investigation.
All of this has been alarming to neighbors.
"It's so devastating, honestly, because at first you think, it was just a fire, and then right before I left work today, I saw that, no, it was a murder," said Aida Lopez, who also lives nearby. "It's devastating that not only the fire but also someone's life was taken... A fire is one thing, but murder? That's a whole different level right there."
State marshal and town investigators were on the scene overnight into Tuesday morning.
The medical examiner is still working to identify the victim. How he is connected to the apartment that burned was not immediately clear.