Robert E. Grimmo III, 22, was taken into custody Monday night after an hours-long manhunt, Highland Park troopers said.
Highland Park Police Chief Lou Jogmen said an officer briefly chased Grimeau, who was driving about five miles north of the shooting, before the man was stopped and taken into custody “without incident.”
Police declined to identify Grimo as a suspect, referring to the teenager as a “person of interest” in the case.
Authorities have not released a possible motive for the shooting.
Lake County Police spokesman Christopher Covelli told a news conference that “multiple casualties” died at the scene and one died at a hospital.
Brigham Temple, director of emergency services at Northshore University Health Center Hospital, where 26 people were injured in the shooting, said at least four were children under the age of eight.
“It is devastating that America’s celebration has been destroyed by the plague,” Illinois Governor JP Pritzker said at a conference.
“I’m angry because it doesn’t have to be this way … we only celebrate the 4th of July once a year, and mass shootings have become a weekly — yes, weekly — American tradition,” he added.
US President Joe Biden, who was “shocked” by the shooting, reiterated that he “will not give up the fight against the epidemic of gun violence”.