The 21-year-old man arrested for a mass shooting at a July Fourth parade in a Chicago suburb has confessed and told police he considered a second attack while on the run.

After fleeing the shooting scene in Highland Park, Illinois, Robert E Crimo III drove to nearby Madison, Wisconsin, where he "seriously contemplated" carrying out another attack, police spokesman Christopher Covelli said.

Assistant State Attorney Ben Dillon said during a bond hearing for Mr Crimo that he had voluntarily confessed to police that he carried out the shooting in Highland Park, which left seven people dead and dozens injured.

Judge Theodore Potkonjak ordered Mr Crimo to be held without bail.

Mr Crimo is accused of spraying gunfire into a parade from a Chicago-area rooftop, turning a holiday celebration into a killing field.

He made his first court appearance today to face seven first-degree murder charges.

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Authorities said Mr Crimo, planned the attack for weeks before climbing up to his sniper's perch from an alley and firing more than 70 rounds randomly into parade spectators before making his getaway dressed as a woman.

Seven people were killed and more than three dozen were treated in hospital for gunshot wounds and other injuries after the carnage in Highland Park, Illinois, a Chicago suburb with a large Jewish community.

The suspect was arrested later on Monday when he was stopped by police at the wheel of his mother's car, which authorities said he had driven to Wisconsin and back to Illinois following the shooting.

If convicted as charged on seven counts of first-degree murder, he would face a mandatory life prison sentence without the possibility of parole, Eric Reinhart, the state's attorney for Lake County, said in announcing the charges yesterday.

"These are just the first of many charges that will be filed against Mr Crimo. I just want to emphasize that," the prosecutor told reporters.

"We anticipate dozens of more charges centered around each of the victims."

Mr Reinhart said he would ask a judge during a court hearing expected today to order Mr Crimo to remain in pretrial detention without bond. It was not immediately clear if Mr Crimo had a lawyer.

Police said they had no immediate evidence of any anti-Semitic or racist basis for the attack.

Investigators were reviewing videos Mr Crimo had posted on social media containing violent imagery.

A high-powered AR-15-type of rifle used in the shooting was found at the scene, and the suspect had a similar weapon in his mother's car when arrested, according to Sergeant Chris Covelli, a spokesman for the Lake County Sheriff's office.

Mr Crimo's parents said in a statement released by their lawyer that they are requesting their privacy as they sort through the tragedy.

"We are all mothers and fathers, sisters and brothers, and this is a terrible tragedy for many families, the victims, the paradegoers, the community, and our own," they said. "Our hearts, thoughts, and prayers go out to everybody."

Red flag law falls short

Mr Covelli said the suspect had legally purchased five guns in all, rifles and pistols, despite having come to law enforcement's attention on two prior occasions for alleged behavior suggesting he might harm himself or others.

The first instance was an April 2019 emergency-911 call reporting Mr Crimo had attempted suicide, followed in September of that year by a police visit regarding alleged threats "to kill everyone" that he had directed at family members, Mr Covelli said.

Police responding to the second incident seized a collection of 16 knives, a dagger and a sword amassed by Mr Crimo in his home, though no arrest was made as authorities at the time lacked probable cause to take him into custody, Mr Covelli said.

"There were no complaints that were signed by any of the victims," Mr Covelli said.

Still, authorities were at a loss to explain precisely how Mr Crimo had evidently slipped past the safeguards of an Illinois "red flag" law designed to prevent people deemed to have violent tendencies from getting guns.

The Illinois State Police said in a separate statement that the knives were returned to the family later the day they were confiscated after the father told authorities they were his and stored in his son's closet for safekeeping.

Nevertheless, state police acknowledged having received a report from local authorities declaring Mr Crimo a "clear and present danger" after the alleged September 2019 threat to his family.

But state police said they closed the matter after determining that Mr Crimo at that time had neither a gun-owner's ID card to revoke nor an application to deny.

Neither of the 2019 incidents surfaced in the four background checks conducted during his subsequent firearms purchases, state police said.

Hearts with the names of victims set by the Highland Park Presbyterian Church

Among those killed in Monday's attack were Nicholas Toledo-Zaragoza, 78, a grandfather from Mexico celebrating with his family among the flag-waving crowds, and Irina McCarthy, 35, and her husband, Kevin McCarthy, 37, the parents of a two-year-old found wandering alone after the attack.

The other dead included Jacki Sundheim, 63, a teacher at a nearby synagogue, Katherine Goldstein, 64, and Stephen Straus, 88.

Police confirmed yesterday that a seventh victim had died.

The bloodshed in Illinois followed a recent string of deadly mass shootings that have renewed debate about gun safety in America.

They included an attack in which 19 school children and two teachers were killed in Uvalde, Texas, on 24 May just ten days after ten people were slain in a supermarket in a predominately black neighborhood in Buffalo, New York.

'Planned attack'

Earlier, Mr Covelli said no motive had been established for the attack, which sent panicked parade-goers fleeing for their lives.

"We do believe Crimo pre-planned this attack for several weeks," and that he acted alone, he said.

"We have no information to suggest at this point it was racially motivated, motivated by religion or any other protected status," he added.

Sgt Covelli said Mr Crimo used a fire escape to access the roof of a building overlooking the parade route and fired more than 70 rounds from a rifle "similar to an AR-15," one of several guns he had purchased legally.

"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," he said.

He said Mr Crimo went to his mother's nearby home after the shooting and borrowed her car. He was captured about eight hours later after a brief chase.

He also said the authorities were investigating disturbing online posts and videos made by Mr Crimo.

Averted shooting in Virginia

Meanwhile, Police in Richmond, Virginia have said they had foiled a potentially disastrous mass shooting during the city's 4 July celebrations.

Richmond Police Chief Gerald Smith said police were tipped off by a member of the public who overheard a conversation on a plan to attack Dogwood Dell, the large city park where thousands were to gather for concerts and fireworks on the US Independence Day holiday.

The call led police on 1 July to a residence where they found one of two men potentially involved, with two assault rifles, a handgun, multiple ammunition clips, and 223 rounds of ammunition.

The person, identified as a non-US citizen, Julio Alvardo-Dubon, aged 52, was arrested on firearms charges.

The second suspect, Rolman Balacarcel, aged 38, was tracked and eventually arrested yesterday on the same charges.

"Their intent was to conduct a mass shooting at our 4 July celebrations," Mr Smith told reporters.

He said authorities did not yet know the motive of the alleged planned attack.

Mr Smith praised the unidentified person who tipped off police in Richmond, around 153 kilometres from the US capital Washington.