Chicago police are trying to determine exactly what happened.
The shooting took place in the 5300-block of North Delphia on the city's Far Northwest Side.
Neighbors inside the condo building say they didn't hear any gunshots and can't understand how this all happened.
ABC7 has learned what happened inside is a tragic accident. Sources say a 78-year-old retired policeman shot his 47-year-old son, Michael Griffin, in the head, mistaking him for an intruder.
Neighbor Michael M. described the father as an ex-military man with many firearms, and said he would often see him around the building, but says he didn't hear a thing coming from below.
Kathy Matzaris lives in the unit across from the father and said he and his son, who recently moved in, were always kind to her, and she too didn't hear any gunshots at all, but police came knocking on her door at 1 a.m.
Others in this building agree, the father was quiet and kept to himself.
The police union did confirm this was an accident.
Police say they are still investigating. The father has not been officially identified or charged.
A friend of the family says, after the shooting, the father had a heart attack and is recovering in the hospital.