Six people were killed have been killed and at least two dozen injured after a man with a high-powered rifle opened fire from a rooftop at a Fourth of July parade in the wealthy Chicago suburb of Highland Park.
Officials said a rifle was recovered from the scene, and a hospital official reported 31 people were injured, an increase on the initial count of 24.
Police said they were searching for a white male about 18 to 20 years old and asked the public for tips and mobile phone images and surveillance video.
"We are aggressively looking for the individual who's responsible," sheriff's department spokesman Christopher Covelli said.
"He could be in the city. He could be somewhere else."
The investigation has made "significant progress," he said without elaborating, except to say the shooting took place from the rooftop of a business the gunman reached via an unsecured alley ladder attached to the building.
The incident comes with gun violence fresh in the minds of many Americans, after a massacre on May 24 killed 19 children and two teachers at a primary school in Uvalde, Texas, and the May 14 attack that killed 10 people at a grocery store in Buffalo, New York.
President Joe Biden said he and his wife Jill were "shocked by the senseless gun violence that has yet again brought grief to an American community on this Independence Day".
Biden said he had "surged federal law enforcement to assist in the urgent search for the shooter."
In his statement, Biden referred to bipartisan gun-reform legislation he signed recently but said much more needed to be done.
"I'm not going to give up fighting the epidemic of gun violence," he said.
Staff at NorthShore Highland Park Hospital in Highland Park were treating 26 people, while five were transported to NorthShore Evanston Hospital.
"The vast majority suffered gunshot wounds," spokesman Jim Anthony said.
Children waving US 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, video on social media showed.
One mobile phone video, seen but not verified by Reuters, recorded what sounded to be about 30 rapid shots, a pause, and then another roughly 30 shots.
Between the two bursts, a woman can be heard saying from the side of the parade route: "My God, what happened?".
Witness Amarani Garcia, who was at the parade with her young daughter, told the local ABC affiliate she heard gunfire, then a pause for what she suspected was reloading, and then more shots.
There were "people screaming and running. It was just really traumatising", 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 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.
A 36-year-old native of Highland Park who wanted to be identified only as Sara, told Reuters she had attended the annual parade most years since childhood.
"Not even five minutes after, very shortly after, the police and firetrucks part of the parade had gone by I heard 'pop, pop, pop, pop, pop'," she said, adding that she first thought they were muskets some times used in parades.
"I looked and there were no muskets. The popping didn't stop ... again it went 'pop, pop, pop, pop, pop' and I turned and I said 'those are gun shots, run'."
Highland Park's population is 30,000 and nearly 90 per cent white, according to the US Census Bureau. About a third of the population is Jewish, according to the Jewish Telegraphic Agency.
At least one of those killed was a Mexican national, a senior Mexican Foreign Ministry official said on Twitter.
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