The six-year-old found the loaded revolver on top of the refrigerator and shot 3-yera-old Eian Santiago in the head on Saturday evening, according to police and the Cook County Medical Examiner's office.
Eian suffered a fatal gunshot wound to his head on and died in hospital in Chicago.
Their father Michael Santiago, 25, was arrested and charged with a child endangerment felony and appeared in court yesterday.
"The gun was purchased off the street. It was kept loaded, and it was wrapped in pajama pants on top of the refrigerator," prosecutor Joseph DiBella was quoted as saying by The Chicago Tribune.
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Santiago, doesn't have a firearm owner's identification card or concealed carry license, DiBella said.
"This is the ultimate tragedy. And whether I said a USD 1 million bond or a lower bond, it's not going to bring back this child.
"... I'm sure the defendant did not intend for this to happen, but it happened. And it's what happens when people have guns who shouldn't have guns. That's why we've had 2,300 people shot in Chicago so far this year," Brown said.