The wife of Cornell McKinney alleges in the suit that off-duty Robbins police officer Christopher Lloyd drove to the couple's South Side home in February and shot McKinney 24 times with an automatic weapon.
Chicago police investigated the shooting but have not charged Lloyd.
The suit claims a medical examiner's report contradicts the police investigation's account of the incident.
"I don't understand, my frustration is that nothing has been done and his killer is out there walking freely around," said Nicole McKinney, victim's wife.
Chicago police say their investigation into the incident remains open.
Robbins police have not commented on the suit and ABC7 has not been able to get a comment from officer Lloyd either.