The gunman accused of attacking an Independence Day parade in suburban Chicago has been charged with seven counts of first-degree murder, a prosecutor said.

Lake County State’s attorney Eric Rinehart promised that dozens more charges would be sought. He added that the suspect, if convicted of murder, would receive a mandatory life sentence without the possibility of parole.

It comes after police said Robert E Crimo III legally bought two high-powered rifles and three other weapons despite authorities being called to his home twice since 2019 after he threatened suicide and violence.

A spokesman for the Lake County Major Crime Task Force told a news conference that the suspected shooter, who was arrested late on Monday, used a high-powered rifle “similar to an AR-15″ to spray more than 70 rounds from atop a commercial building into a crowd that had gathered for the parade in Highland Park, an affluent community of about 30,000 on the Lake Michigan shore.

Police said they were called to the suspect’s home in September 2019 after a family member called to say he was threatening “to kill everyone” in the home.

Task force spokesman Christopher Covelli said police confiscated 16 knives, a dagger and a sword, but said there was no sign he had any guns at the time.

The suspect legally purchased the high-powered rifle in Illinois within the past year, Mr Covelli said. In all, police said, he purchased five firearms, which were recovered by officers at his father’s home.

Police in 2019 also responded to a reported suicide attempt by the suspect, Mr Covelli said.

The day after the shooting, authorities reported the death of a seventh person. More than three dozen other people were wounded in the attack, which Mr Covelli said the suspect had planned for several weeks.

Investigators who have interrogated the suspect and reviewed his social media posts have not determined a motive for the attack or found any indication that he targeted victims by race, religion or other protected status, Mr Covelli said.

Earlier in the day, FBI agents peeked into trash cans and under picnic blankets as they searched for more evidence at the site where the assailant opened fire. The shots were initially mistaken for fireworks before hundreds of revellers fled in terror.

The 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.

A police officer pulled over Crimo north of the shooting scene several hours after police released his photo and warned that he was likely armed and dangerous, Highland Park Police Chief Lou Jogmen said.

Law enforcement officials gave his age as 21 or 22.

After evading initial capture by dressing as a woman and blending into the fleeing crowd, Crimo drove to the Madison, Wisconsin, area, then returned to Illinois, Mr Covelli said.

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

Among them was Nicolas Toledo, who was visiting his family in Illinois from Mexico. He was shot and died 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.

The Lake County coroner released the names of four other victims: 64-year-old Katherine Goldstein, 35-year-old Irina McCarthy, 37-year-old Kevin McCarthy and 88-year-old Stephen Straus.

Nine people, ranging from 14 to 70, remained in hospital on Tuesday, officials said.