Police said they had no immediate information on a possible motive for the shootings, but said the victims were believed to be members of the same family.
Local sheriff's deputies and agents for the FBI's fugitive task force were assisting in the manhunt, Joilet Police Chief William Evans said at a press briefing.
The suspect was identified by the Joilet Police Department as 23-year-old Romeo Nance, whom authorities said should be considered armed and dangerous.