The 22-year-old man identified as a person of interest in the July 4 parade shooting in suburban Chicago has been taken into custody, police said.

Highland Park Police chief Lou Jogmen earlier said Robert E Crimo III should be considered armed and dangerous and was pulled over by police on Monday evening after a brief pursuit.

At least six people were killed and 30 wounded after a gunman on a rooftop opened fire on an Independence Day parade in suburban Chicago, sending hundreds of marchers, parents with strollers and children on bicycles fleeing in terror.

The suspect remained on the loose for hours after the incident as authorities scoured the area and police surrounded a home listed as his possible address.

The July 4 shooting was just the latest to shatter the rituals of American life. Schools, churches, grocery stores 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.

"It is devastating that a celebration of America was ripped apart by our uniquely American plague," Illinois Governor JB Pritzker said at a news conference.

"I'm furious because it does not have to be this way... while we celebrate the Fourth of July just once a year, mass shootings have become a weekly - yes, weekly - American tradition."

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

Dozens of fired bullets sent hundreds of parade-goers - some visibly bloodied - fleeing. They left a trail of abandoned items that showed everyday life suddenly, violently disrupted: A half-eaten bag of potato chips; a box of chocolate cookies spilled onto the grass; a child's Chicago Cubs cap.

Lake County Major Crime Task Force spokesman Christopher Covelli said at a news conference "several of the deceased victims" died at the scene and one was taken to a hospital and died there. Police have not released details about the victims or wounded.

Lake County coroner Jennifer Banek said the five people killed at the parade were adults, but did not have information on the sixth victim who was taken to a hospital and died there.

One of those killed was a Mexican national, Roberto Velasco, Mexico's director for North American affairs, said on Twitter on Monday. He said two other Mexicans were wounded.

NorthShore University Health Centre received 26 patients after the attack. All but one had gunshot wounds, said Dr Brigham Temple, medical director of emergency preparedness. Their ages ranged from 8 to 85, and Dr Temple estimated that four or five patients were children.

Dr Temple said 19 of them were treated and discharged. Others were transferred to other hospitals, while two patients, in stable condition, remained at the Highland Park hospital.

The gunman opened fire around 10.15 am, when the parade was about three-quarters through, authorities said.

Highland Park Police commander Chris O'Neill, the incident commander on scene, said the gunman apparently used a "high-powered rifle" to fire from a spot atop a commercial building where he was "very difficult to see".

He said the rifle was recovered at the scene. Police also found a ladder attached to the building.

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

President Joe 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".

Mr Biden 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.

Highland Park is a close-knit community of about 30,000 people located on the shores of Lake Michigan just north of Chicago, with mansions and sprawling lakeside estates that have long drawn the rich and sometimes famous, including NBA legend Michael Jordan, who lived in the city for years when he played for the Chicago Bulls.

Mr Pritzker, a Democrat, promised support for the community as well as to bring the gunman to justice.

"There are no words for the kind of evil that shows up at a public celebration of freedom, hides on a roof and shoots innocent people with an assault rifle," Mr Pritzker said.