Gunshots rang out Monday at a US National Day parade in an affluent Chicago suburb.
At least five people died and others were injured.
A suspect is still on the run.
The Lake County Sheriff's Office called on Twitter to avoid the Highland Park area.
The mayor of the city of Chicago said downtown should also be avoided.
According to the Chicago Sun-Times, the shots were fired about 10 minutes after the parade began.
Local television stations reported people running away screaming or seeking cover.
Several eyewitness videos are circulating on the internet.
Brad Schneider, who represents the state of Illinois in the House of Representatives, was in the Lake Michigan city when the shots rang out.
"My team and I had just met at the beginning of the parade when the shots were fired," Schneider said on Twitter after he fled to safety.
The 4th of July festival has been cancelled.
Every year, Americans celebrate the ratification of their country's Declaration of Independence on July 4, 1776.