The 21-year-old suspect also told authorities that he had contemplated carrying out a second shooting.
The man charged with killing seven people at a July 4 shooting in Highland Park has reportedly confessed to police.
The 21-year-old suspect -- identified as Robert Crimo III -- also told authorities he had contemplated carrying out a second shooting, prosecutors said on Wednesday.
Crimo had fled to Madison, Wisconsin, after shooting at an Independence Day parade from a roof in suburban Chicago.
More than two dozen people were wounded, some critically, and hundreds of marchers, parents and children fled in a panic. Some of the wounded remained hospitalised in critical condition and the death toll could still rise.
The suspect initially evaded capture by dressing as a woman and blending into the fleeing crowd. He was then arrested as he turned back to Illinois after deciding against another shooting, according to Lake County Major Crime Task Force spokesman Christopher Covelli.
Covelli said that it did not appear that Crimo had planned another attack in Wisconsin, but “seriously contemplated” firing on another July 4 celebration he saw there.
The gunman had thrown away the rifle he used in Illinois, but had another weapon and around 60 more rounds of ammunition, according to Covelli.
Investigators who have interrogated the suspect and reviewed his social media posts have not determined a motive or found any indication that he targeted victims by race, religion or other protected status, he adde.
Questions have also arisen about how the suspect had legally obtained five weapons given Illinois’ relatively strict gun laws.
Authorities had been called to his home twice in 2019 for threats of violence and suicide and had confiscated 16 knives, a dagger, and a sword.
Lake County State’s Attorney Eric Rinehart said he planned to bring attempted murder and aggravated battery with a firearm charge for each individual hurt.
“There will be many, many more charges coming,” he said at a news conference on Wednesday.