Highland Park gunman Robert E. Crimo III was dressed in women's clothing during the attack in order to blend into the fleeing crowd, police said on Tuesday.
"Investigators do 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."
Crimo, who police believe preplanned the attack, fired more than 70 rounds at the Independence Day parade. Police confirmed that the shooter purchased the gun legally in Illinois, adding that the victims appear to have been targeted at random.
"We do believe Crimo pre-planned this attack for several weeks," and that he acted alone, police spokesman Christopher Covelli said. "We have no information to suggest at this point it was racially motivated, motivated by religion or any other protected status."
Covelli said Crimo went to his mother's nearby home after the shooting spree and borrowed her car.
Authorities detained Crimo late Monday at a traffic stop after a brief chase. He turned out to be an aspiring Chicago-based rapper performing under the name 'Awake the Rapper.' Crimo is said to have posted "multiple disturbing videos on YouTube with violent imagery" that included a man with a rifle shooting people.
Six people were killed and dozens wounded in the shooting during an Independence Day parade in Highland Park on Monday.