The July Fourth parade in the Chicago suburb of Highland Park ended on a fatal note on Monday.
Six people, among the 30, shot at about 10:14 am, have been confirmed dead.
Security agents are searching for the suspect believed to have opened fire from a rooftop.
Highland Park Police Commander Chris O’Neill advised people to stay indoors until the general area is declared safe.
Lake County Major Crime Task Force spokesman Christopher Covelli told reporters that the gunman’s rifle was recovered at the scene.
Mayor Nancy Rotering lamented that the community was “terrorized by an act of violence that has shaken it to the core”.
Rotering told a news conference that instead of celebrating freedom, the people are now mourning the tragic loss of life and struggling with the terror.
“Our hearts go out to the families of the victims at this devastating time”, Rotering added.
Illinois Governor Jay Robert Pritzker condemned the shooting in a statement.
Pritzker said there are no words for a “monster who lies in wait and fires into a crowd of families with children celebrating a holiday”.
