The suspected gunman who opened fire on a July 4 parade in a wealthy Chicago suburb planned the attack for weeks and wore women's clothing to aid his escape, police said Tuesday.

Robert Crimo, 21, whose family lives in Highwood, just north of Highland Park, where the shooting occurred, was arrested Monday after six people were killed and more than 30 wounded during an Independence Day parade.

Deputy police chief Christopher Covelli said no motive had been established for the attack, in which the gunman sprayed the crowd with dozens of semi-automatic rounds fired from a rooftop.

"We do believe Crimo pre-planned this attack for several weeks," Covelli said.

"He brought a high-powered rifle to this parade, he accessed the roof of a business via a fire escape ladder and began opening fire.

"Crimo was dressed in women's clothing and investigators believe he did this to conceal his facial tattoos and his identity and help him during the escape with the other people who were fleeing the chaos."

Covelli said the gunman fired more than 70 rounds from a rifle "similar to an AR-15" that he had purchased legally.

Authorities are investigating disturbing online posts and videos made by Crimo, who was captured about eight hours after the shooting following a brief car chase.

The shooting has left the upscale suburb in shock.

"We're all still reeling," Mayor Nancy Rotering told NBC's Today show.

"Unbelievable shock," Rotering said. "Everybody knows somebody who was affected by this directly."

The mayor said she personally knew the suspected gunman when he was a young boy in the Cub Scouts and she was a Cub Scout leader.

"How did somebody become this angry, this hateful to then take it out on innocent people who literally were just having a family day out?" Rotering asked.

- Violent online content -

David Baum, a doctor whose two-year-old was in the parade, witnessed the shooting and helped treat some of the injured.

"The people who were gone were blown up by that gunfire," Baum told CNN, calling it a "horrific scene."

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 July 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.