Police are hunting for a lone gunman after at least six people were killed and dozens of others wounded in an attack on families celebrating Independence Day in Chicago.

The man, identified by police as 22-year-old Robert E. “Robb” Crimo III,  allegedly perched on a roof and took shots at people attending the Highland Park Independence Day parade.

Children were among the victims.

Residents of Highland Park have been warned to remain on “high alert” while police hunt the suspect.

Investigators from the City of Highland Park Police Department released a photo of the suspect on Tuesday morning.

A person of interest has been identified as Robert (Bobby) E. Crimo III. Photo: City of Highland Park Police Department

Police found a “high-powered” rifle the killer allegedly ditched at the scene. It’s not known whether he was armed with additional weapons.

It’s believed the gunman climbed up a ladder and stood on the roof of a building up an alleyway before firing at the crowd.

“He was very discreet and very difficult to see,” said Lake County Sheriff’s office Deputy Chief, Christopher Covelli.

Witnesses have described how they ran for their lives alongside hundreds of other parade-goers, with parents explaining they hid their children in garbage bins and behind cars.

Ariel Rotbol said it was “a war scene”.

“People just in hysteria. We saw the helicopter, the cars, Homeland Security coming in,” she told ABC 7 Chicago. 

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Chicago Sun-Times journalist Lynn Sweet, who was attending the parade, described the shooting as a “bloodbath”. Her photographs from the scene showing bodies laying across the pavement.

“It sounded like a string of about 20 firecrackers that were led off inside of your mental garbage bin,” another witness, Jeff Leon, told CNN.

“So, I didn’t immediately react, I thought it was just how people are on the fourth.

“We just took off… we were hiding behind cars, folding into the next car and making our way…but then we had to find our kids.”

Chicago police will have the help of the FBI. Photo: Getty

Another witness, former Sun-Times journalist Adrienne Drell, said she was watching the parade when members of the Highland Park High School marching band started running down the street. One man picked her up off the kerb and told her to take cover, the Sun-Times reported.

“There’s panic in the whole town,” Ms Drell told the paper.

“Everyone is just stunned beyond belief.

“It was a quiet, peaceful, lovely morning, people were enjoying the parade…within seconds, to have that peacefulness suddenly ripped apart, it’s scary.”

She added: “You can’t go anywhere, you can’t find peace. I think we are falling apart.”

Highland Park Mayor Nancy Rotering said the shooter had “terrorised” the community and that the tragedy had “shaken us to our core”.

“Please contact your loved ones and ensure that they’re safe and let them know that you’re safe as well,” Ms Rotering said.

“On a day that we came together to celebrate community and freedom, we’re instead mourning the tragic loss of life and struggling with the terror that was brought upon us.”

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Illinois Governor JB Pritzker 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”.

“There are no words for the kind of evil that robs our neighbours of their hopes, their dreams, their futures,” Mr Pritzker said.

“There are no words I can offer to lift the pain of those they leave behind. Please know that our state grieves with you.”

The shooting comes with gun violence fresh on the minds of many in the United States after a massacre on May 24 killed 19 school children and two teachers at an elementary school in Uvalde, Texas.

That followed the May 14 attack that killed 10 people at a grocery store in Buffalo, New York.