July 4 gunman planned attack for weeks and wore women's clothes: police
US-NEWS-CHICAGO-PARADE-SHOOTING-CRIMO-MCT. - City of Highland Park Police Department/City of Highland Park Police Department/TNS

US police said Tuesday that the suspected gunman who opened fire at a July 4 holiday parade in a Chicago suburb had planned the attack for weeks and wore women's clothes to try to cover up his facial tattoos.

Police confirmed that the alleged shooter, Robert "Bobby" Crimo, 21, bought the gun legally, adding that the victims of the attack appeared to have been targeted at random.

"We do believe Crimo pre-planned this attack for several weeks. He brought a high-powered rifle to this parade," police spokesman Chris Covelli told reporters.

"He accessed the roof of a business via a fire escape ladder and began opening fire.

"Crimo was dressed in women's clothing, and investigators do believe he did this to conceal his facial tattoos and his identity during his escape."

Six people were killed and more than two dozen injured in the shooting, in which Crimo fired more than 70 rounds. He was arrested hours after the attack.

"Crimo exited the roof, he dropped his rifle and he blended in with the crowd," Covelli said.