A gunman opened fire on an Independence Day parade in suburban Chicago on Monday, killing at least six people, wounding 24 and sending hundreds of marchers, parents with buggies and children on bicycles fleeing in terror, police said.

Highland Park police chief Lou Jogmen said they have identified 22-year-old Robert E. Crimo as a person of interest and said he should be considered armed and dangerous.

Highland Park police commander Chris O’Neill, the incident commander at the scene, urged people to shelter in place as authorities continue their search.

The July 4 shooting was just the latest to shatter the rituals of American life.

Schools, churches, grocery shops and now community parades have all become killing grounds in recent months.

This time, the bloodshed came as the nation tried to find cause to celebrate its founding and the bonds that still hold it together.

Mayor Nancy Rotering said the violence “has shaken us to our core”, adding, “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.”

The shooting happened at a spot on the parade route where many residents had staked out prime viewing points early in the day for the annual celebration.

But dozens of bullets sent hundreds of parade-goers, some visibly bloodied, fleeing.

They left a trail of abandoned items that showed everyday life suddenly, violently disrupted.

Lake County major crime taskforce spokesman Christopher Covelli said “several of the deceased victims” died at the scene and one was taken to a hospital and died there.

Authorities said the gunman opened fire at about 10.15am, when the parade was about three-quarters through.

Mr Covelli said at a news conference that the gunman apparently used a “high-powered rifle” to fire from a spot on the top of a building where he was “very difficult to see”. He said the rifle was recovered at the scene.

“Very random, very intentional and a very sad day,” Mr Covelli said.

President Joe Biden last month signed the widest-ranging gun violence bill passed by Congress in decades, a compromise that showed at once both progress on a long-intractable issue and the deep-seated partisan divide that persists.

Mr Biden on Monday said he and first lady Jill Biden were “shocked by the senseless gun violence that has yet again brought grief to an American community on this Independence Day”.

He said he had “surged Federal law enforcement to assist in the urgent search for the shooter, who remains at large at this time”.

Police believe there was only one gunman but warned that he should still be considered armed and dangerous.

Several nearby cities cancelled events including parades and fireworks, some of them noting that the Highland Park gunman was still at large.

“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,” Mr Covelli 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.”

More than 100 law enforcement officers were called to the parade scene or sent to find the gunman.

Hours after the shooting, law enforcement officers searched an office building near where the shooting occurred.

Nearby, armed FBI agents in camouflage escorted a family with two small girls across Central Street hours after the shooting.

The children looked visibly frightened even as their mother attempted to reassure them that the agents leading and flanking them would protect them.

“Don’t worry, you’re safe now,” she told them. “These guys will protect you.”

Ominous signs of a joyous event suddenly turned to horror filled both sides of Central Street where the shooting occurred.

Dozens of baby buggies, some bearing US flags, abandoned children’s bikes, a helmet with images of Cinderella, were left behind in their haste.

Blankets, lawn chairs, coffees and water bottles were knocked over as people fled.

Illinois governor J.B. Pritzker said in a statement: “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.”

Gina Troiani and her son were lined up with his daycare class ready to walk on to the parade route when she heard a loud sound that she believed was fireworks – until she heard people shout about a gunman.

“We just start running in the opposite direction,” she told The Associated Press.

Her five-year-old son was riding his bike decorated with red and blue curled ribbons. He and other children in the group held small flags. The city said on its website that the festivities were to include a children’s bike and pet parade.

Ms Troiani said she pushed her son’s bike, running through the neighbourhood to get back to their car.

In a video that she shot on her phone, some of the children are visibly startled at the loud noise and they scramble to the side of the road as a siren wails nearby.

It was just sort of chaos,” she said. “There were people that got separated from their families, looking for them. Others just dropped their wagons, grabbed their kids and started running.”

Debbie Glickman, from Highland Park, said she was on a parade float with co-workers and the group was preparing to turn on to the main route when she saw people running from the area.

“People started saying: ‘There’s a shooter, there’s a shooter, there’s a shooter,’” she told the Associated Press. “So we just ran. We just ran. It’s like mass chaos down there.”

“I’m so freaked out,” she said. “It’s just so sad.”