Crimo had planned the attack for weeks, officials said on Tuesday.
They said he fired more than 70 rounds at random into the crowd watching Monday's parade, and that he was dressed in women's clothes to help conceal his identity and blend in with the panic-stricken crowd as he fled.
"He blended right in with everybody else as they were running around, almost as if he was an innocent spectator as well," said Sergeant Chris Covelli, a spokesperson for the Lake County Sheriff's office, adding that the suspect has distinctive facial tattoos.
In addition to the seven victims killed by gunfire, more than three dozen people were treated in hospitals for gunshot wounds and other injuries.
Covelli said Crimo had two previous encounters with law enforcement - an April 2019 emergency-911 call reporting that he had attempted suicide and another in September of that year regarding alleged threats "to kill everyone" that he had directed at family members.
Police responding to the second incident seized a collection of 16 knives, a dagger and a sword amassed by Crimo in his home, though no arrest was made as authorities at the time lacked probable cause to take him into custody, Covelli said.
"There were no complaints that were signed by any of the victims," Covelli said.
But a state "red flag" system, designed to allow police to seek a court order to seize weapons from people who are deemed to present a danger to themselves or others, appeared to have broken down.
Among those killed in Monday's attack were Nicholas Toledo, a grandfather from Mexico in his 70s celebrating with his family among the flag-waving crowds, and Jacki Sundheim, a teacher at a nearby synagogue.
The shooting took place in a neighborhood with a large Jewish population, but police had no immediate evidence of any anti-Semitic or racist basis. Investigators were reviewing videos Crimo had posted on social media containing violent imagery.
The suspect used a high-powered rifle for the attack, similar to an AR-15, which he dropped at the scene.
He had a similar rifle in his mother's car, which he was driving when police took him into custody, and owned other guns, all of which were bought legally in Illinois, officials said.
In all, Crimo had purchased five firearms, including rifles and pistols.
Highland Park Mayor Nancy Rotering said the community of 30,000 was in shock.
"This tragedy should have never arrived at our doorsteps," she told NBC News. "As a small town, everybody knows somebody who was affected by this directly and, of course, we are all still reeling."