"There will be more charges," Lake County State's Attorney Eric Rinehart told reporters. "We anticipate dozens of more charges centered around each of the victims."
Police spokesman Christopher Covelli said the death toll rose to seven on Tuesday after one of the victims died in hospital. More than 35 people were wounded.
Among the dead were Kevin McCarthy, 37, and his wife, Irina, 35 -- the parents of a two-year-old boy who was found wandering alone after the shooting, according to CBS News.
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.
He said Crimo has a history of mental health issues and threatening behavior.
Police had been called to Crimo's home twice in 2019: once to investigate a suicide attempt and the second time because a relative said he had threatened to "kill everyone" in the family, he said.
Police removed 16 knives, a dagger and a sword from the home but did not make any arrests, he said.
Covelli said 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.
'Still reeling'
Covelli said 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 Crimo.
The shooting has left the upscale suburb in shock.
"We're all still reeling," Mayor Nancy Rotering told NBC's Today show. "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.
"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?"
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.
"I need to just do it," a voice-over says.
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" hat in numerous photos and 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 about 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 a New York supermarket, and 19 children and two teachers slain at an elementary school in Texas.
Vice President Kamala Harris, who was in Chicago Tuesday for a summit of the nation's largest teachers' union, said the Texas shooting was a reminder "of the risks that our children and our educators face every day," and renewed a call for Congress to ban assault weapons.
Speaking later at the scene of the Highland Park shooting, Harris said: "The whole nation should understand... that this could happen anywhere, in any peace-loving community."
In another July 4 shooting, two police officers were wounded when they came under fire during a fireworks show in Philadelphia, officials said.
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.
Cassie Goldstein, another survivor of the attack, told local media she had seen her mother die as they fled the shooting.
"I started running with her, we were next to each other, and he shot her in the chest, and she fell down and I knew she was dead," the 22-year-old told NBC News.
"So I just told her that I loved her, but I couldn't stop because he was still shooting everyone next to me."
Francisco Toledo, whose 78-year-old father Mario was killed in the shooting, told AFP he felt the gunman had been "deceived... by an evil spirit."
"I have been asked questions: 'What would you do if you had him here in front of you?' I wouldn't ask him anything, I would tell him to repent," he said.
President Joe Biden vowed to keep fighting "the epidemic of gun violence."
Last week, he 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.
