At least 6 people were killed and several more were injured in a shooting at a Fourth of July parade in the wealthy Chicago suburb of Highland Park on Monday, officials said, as panicked spectators fled the scene.
The city of Highland Park reported on its website that 6 people were dead and 24 transferred to hospital.
"Numerous law enforcement officers are responding and have secured a perimeter around downtown Highland Park," the statement said.
Officials told a news conference that six people were killed and 24 taken to hospital, and that a rifle was recovered from the scene.
Police were searching for a white male, believed to be about 18 to 20 years old, and asked the public for tips to help find him. Police said initial reports suggested he fired from a rooftop.
"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 during this devastating time," Highland Park Mayor Nancy Rotering told the news conference.
One hospital spokesperson said there were 31 injured. Staff at NorthShore Highland Park Hospital in Highland Park were treating 26 people and five were transported to NorthShore Evanston Hospital, spokesperson Jim Anthony said.
"The vast majority suffered gunshot wounds. Remaining individuals sustained injuries as a result of chaos at the parade," Anthony said.
Children waving American flags, riding tricycles or enjoying a ride in a wagon pulled by adults froze as people in the crowd screamed while gun shots rang out, according to a video posted on social media.
Witness Amarani Garcia, who was at the parade with her young daughter, told the local ABC affiliate she heard gunfire nearby, then a pause for what she suspected was reloading, and then more shots again.
Security personnel seen around the site of shooting in Chicago suburb of Highland Park on Monday. AP
There were "people screaming and running. It was just really traumatizing," Garcia said. "I was very terrified. I hid with my daughter actually in a little store. It just makes me feel like we're not safe anymore."
Social media video showed a marching band in the parade suddenly breaking formation and running away, and other images of people leaving their belongings behind as they sought safety.
"Everyone was running, hiding and screaming," said CBS 2 Digital Producer Elyssa Kaufman, who was at the scene, the channel's website reported.
"It sounded like a string of about 20 firecrackers that were let off inside of your mental garbage bin, it was that loud," Jeff Leon, 57, told CNN, estimating the gunfire lasted less than a minute.
"So, I didn't immediately react, I thought it was just how people are on the fourth."
The shooting comes with gun violence fresh on the minds of many Americans, after a massacre on May 24 killed 19 school children and two teachers at an elementary school in Uvalde, Texas, and the May 14 attack that killed 10 people at a grocery store in Buffalo, New York.
Witness Amarani Garcia, who was at the parade with her young daughter, told the local ABC affiliate she heard gunfire nearby, then a pause for what she suspected was reloading, and then more shots again.