The suspect knew the victims, who were found in two homes across the street from each other, say police.
Seven people have been found shot dead in two houses in Illinois, US police say, and a manhunt for a suspected gunman is under way.
Police said they are seeking 23-year-old Romeo Nance in connection with the attack in the city of Joliet.
Joliet police Chief Bill Evans said the suspect knew the victims, who were found in two homes across the street from each other on Monday afternoon.
Police said Mr Nance “should be regarded as armed and dangerous”.
Investigators said he was believed to be driving a red Toyota Camry. They urged anyone with information about the attack, 40 miles (64km) south-west of Chicago, to contact them.