Police have named the man suspected of killing at least six people and seriously injuring dozens more at a Fourth of July parade in a Chicago suburb.
Highland Park police confirmed the suspect is Robert E Crimo III, aged 22.
He is still on the run and believed to be driving a silver Honda car.
The attacker opened fire on the crowd in Highland Park, according to officials.
Residents were urged to stay inside as officers searched for the suspect, who is described as a white man wearing a white or blue T-shirt.
He apparently opened fire from a rooftop using a high-powered rifle, which was recovered at the scene, Lake County Major Crime Task Force spokesman Christopher Covelli said.
Mr Covelli confirmed that police believe there was only one shooter and that he should be considered armed and dangerous.
“You have a tragic mass act of violence that was random here today at a community event where people were gathered to celebrate, and the offender has not been apprehended thus far,” he said.
“So, could this happen again? We don’t know what his intentions are at this point, so certainly we’re not sure of that.
“Very random, very intentional and a very sad day.”
Nancy Rotering, the mayor of Highland Park, said: “This morning at 10.14 our community was terrorised by an act of violence that has shaken us to our core.
“Our hearts go out to the families of the victims at this devastating time.
“On a day that we came together to celebrate community and freedom, we are instead mourning the tragic loss of life and struggling with the terror that was brought upon us.”
According to The Chicago Sun-Times, the parade was stopped after only 10 minutes when shots were fired.
Several witnesses told the newspaper they heard gunfire and that hundreds of parade-goers – some visibly bloodied – fled the route, leaving behind chairs, pushchairs and blankets.
A Sun-Times reporter saw blankets placed over three bloodied bodies.
Debbie Glickman, who lives in Highland Park, told Associated Press she was on a parade float with co-workers when she saw people running away from the area.
“People started saying ‘there’s a shooter, there’s a shooter, there a shooter’.
“So we just ran. We just ran. It’s like mass chaos down there.”
J B Pritzker, the governor of Illinois, said: “There are no words for the kind of monster who lies in wait
and fires into a crowd of families with children celebrating a holiday with their community.”
And Congressman Brad Schneider, who was at the event, tweeted: “My condolences to the family and loved ones; my prayers for the injured and for my community; and my commitment to do everything I can to make our children, our towns, our nation safer. Enough is enough!”
President Joe Biden said in a statement: “Jill and I are shocked by the senseless gun violence that has yet again brought grief to an American community on this Independence Day.
“I recently signed the first major bipartisan gun reform legislation in almost thirty years into law, which includes actions that will save lives.
“But there is much more work to do, and I’m not going to give up fighting the epidemic of gun violence.”