On Tuesday, police and FBI agents were sifting through belongings left behind by members of the crowd as they fled.
Strollers, bicycles, folding chairs and other items littered the parade route through the main street of Highland Park.
Crimo, whose father unsuccessfully ran for mayor and owns a store in Highland Park called Bob’s Pantry and Deli, was an amateur musician billing himself as “Awake the Rapper”.
The younger Crimo’s online postings include violent content that alluded to guns and shootings.
One YouTube video posted eight months ago featured cartoons of a gunman and people being shot.
A voice-over says, “I need to just do it.”
It adds: “It is my destiny. Everything has led up to this. Nothing can stop me, not even myself.”
Crimo, who has the word “Awake” tattooed over an eyebrow, is seen sporting an “FBI” baseball cap in numerous photos and is wearing a Trump flag as a cape in one picture.
The shooting is the latest in a wave of gun violence plaguing the United States, where approximately 40,000 deaths a year are caused by firearms, according to the Gun Violence Archive.
“EPIDEMIC OF GUN VIOLENCE”
The deeply divisive debate over gun control was reignited by two massacres in May that saw 10 black people gunned down at an upstate New York supermarket and 19 children and two teachers slain at an elementary school in Texas.
The Highland Park shooting cast a pall over Independence Day, when towns and cities across the United States hold parades and people attend barbecues, sporting events and fireworks displays.
In another Jul 4 shooting, two police officers were wounded when they came under fire during a fireworks show in Philadelphia.
In Highland Park, Emily Prazak, who marched in the parade, described the mayhem.
“We heard the pop, pop, pop, pop, pop, and I thought it was fireworks,” Prazak said.
Five of the six people killed, all adults, died at the scene. The sixth was taken to hospital but succumbed to wounds there.
Highland Park Hospital, where most of the victims were taken, said it had received more than two dozen people with gunshot wounds aged eight to 85.
President Joe Biden voiced shock and vowed to keep fighting “the epidemic of gun violence.”
“I’m not going to give up,” he said.
Last week, Biden signed the first significant federal bill on gun safety in decades, just days after the Supreme Court ruled that Americans have a fundamental right to carry a handgun in public.