"A total of seven people were found shot to death" in two residences, Bill Evans, chief of police in Joliet, Illinois, told a news conference.
"We are currently looking for a suspect by the name of Romeo Nance," he said, adding that Nance, 23, lived in the immediate vicinity of the shootings.
Police were also searching for a car in relation to what they described as "random" shootings in the area one day earlier that left one man dead and another seriously wounded.
Shootings are common in the United States, a country where there are more guns than people, and where attempts to clamp down on their spread are always met with stiff resistance.
Polls show a majority of voters favor stricter controls on the use and purchase of firearms. However, the gun-ownership lobby is highly influential and courts have ruled that the constitutional right to bear arms applies to private owners.