FBI agents are searching for more evidence at the site where a gunman killed at least six people after opening fired on a US Independence Day parade from a suburban Chicago rooftop.

The shots were initially mistaken for fireworks before hundreds of panicked revellers fled in terror in Highland Park, a close-knit community on the shores of Lake Michigan.

A day later, baby strollers, lawn chairs and other items left behind by panicked paradegoers remained inside a wide police perimeter.

Outside the police tape, some residents drove up to collect blankets and chairs they abandoned.

Authorities detained a suspect – named as Robert E Crimo III – on Monday evening in a traffic stop that led to a brief chase.

Police initially described the man as a person of interest, but a spokesman for the Lake County Major Crime Task Force said that he is now considered a suspect.

Charges are expected to be announced soon, according to a spokeswoman for the Lake County state’s attorney, Sara Avalos.

Authorities offered no motive for the attack.

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 America tried to find cause to celebrate its founding and the bonds that still hold it together.

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.

Among them was Nicolas Toledo, who was visiting his family in Illinois from Mexico. He was shot and killed at the scene, his granddaughter, Xochil Toledo, told the Chicago Sun-Times.

Also killed was Jacki Sundheim, a lifelong congregant and “beloved” staff member at nearby North Shore Congregation Israel, which announced her death on its website.

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: a box of chocolate cookies spilled onto the grass; a child’s Chicago Cubs cap; baby strollers, some bearing American flags.

“There’s no safe place,” said Highland Park resident Barbara Harte, 73, who had stayed away from the parade fearing a mass shooting, but later ventured from her home.

Highland Park Police Chief Lou Jogmen said a police officer pulled over Crimo about five miles north of the shooting scene, several hours after police released the man’s photo and warned that he was likely armed and dangerous.

Authorities initially said Crimo, whose father once ran for mayor of Highland Park, was 22, but an FBI bulletin and Crimo’s social media said he was 21.

Lake County Coroner Jennifer Banek said the five people killed at the parade were adults, but did not have information on the sixth.

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 eight to 85, and Dr Temple estimated that four or five were children.

“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.

“While we celebrate the Fourth of July just once a year, mass shootings have become a weekly — yes, weekly — American tradition.”

Crimo, who goes by the name Bobby, was an aspiring rapper with the stage name Awake the Rapper, posting on social media dozens videos and songs, some ominous and violent.

In one animated video since taken down by YouTube, Crimo raps about armies “walking in darkness” as a drawing appears of a man pointing a rifle, a body on the ground and another figure with hands up in the distance.

Crimo’s father, Bob, a longtime deli owner, ran unsuccessfully for mayor of Highland Park in 2019, calling himself “a person for the people”.

The community of about 30,000 on Chicago’s north shore has mansions and sprawling lakeside estates and was once home to NBA legend Michael Jordan.