Chief Covelli said Mr. Crimo planned the shooting for several weeks, but that the authorities had not yet established what his motive was. “We have no information to suggest at this point it was racially motivated, motivated by religion, or any other protected status,” the chief said, adding that there was no indication that anyone else was involved.
The police had at least two past encounters with the suspect.
People had contacted the police with concerns about Mr. Crimo twice in recent years, Chief Covelli said.
April 2019: Someone contacted the police after learning Mr. Crimo had recently attempted suicide. Chief Covelli said on Tuesday that the police spoke with Mr. Crimo and his parents about the matter at the time, but did not pursue it further because it was being handled by mental health professionals.
September 2019: A member of Mr. Crimo’s family told the police that he had knives and was going to “kill everyone,” Chief Covelli said. Officers who went to the family home removed 16 knives, a dagger and a sword, but the chief said they found no probable cause to make an arrest.
The gunman used a disguise to get away.
The authorities said the gunman wore women’s clothing to disguise his identity. He climbed up a fire escape ladder to gain access to the roof where he staged his attack, and then after the shooting, he climbed back down and left the scene on foot, blending in with fleeing paradegoers.
The police said Mr. Crimo walked to his mother’s home in Highland Park and borrowed her car. He then drove to the Madison area in Wisconsin before returning to Illinois, Chief Covelli said. He was arrested after the police in a nearby town spotted the car and tried to pull him over for a traffic stop.
The attack was one of a number of multiple shootings in recent days.
The mass shooting in Highland Park was the fourth in Illinois since Friday in which at least four people were struck, according to the Gun Violence Archive. The state has among the strictest gun-safety laws in the nation — including universal background checks, red flag warnings and safe storage requirements — but it is surrounded by states like Indiana that have far fewer restrictions to gun purchase and ownership.
Ten hours before the parade shooting, at about midnight, five people were shot at a housing complex in the Greater Grand Crossing neighborhood on Chicago’s South Side. On Friday, two people were killed and seven injured in two separate shootings in Chicago, according to the Gun Violence Archive.
Also on Monday, the group reported, there were shootings with four or more people injured in Boston, Sacramento, Kansas City, Mo., and Richmond, Va.
In the hours after the attack on the parade in Illinois, eruptions of gunfire killed or wounded multiple people in Kenosha, Wis., and Gary, Ind., both within 60 miles of Highland Park.