Highland Park shooting: Man arrested after 4 July mass shooting
Robert E Crimo III, 22, was detained after a brief chase, police said.
The gunman climbed on to a roof, shooting randomly at spectators using a high-powered rifle.
It is the latest mass shooting to hit the US - there has been one in every week of 2022. President Joe Biden said he was "shocked" by the violence.
Hours later, two police officers were wounded in a shooting in Philadelphia during a Fourth of July fireworks display.
Mr Crimo was detained after a manhunt. He was referred to as a "person of interest" in Monday's shooting, but after his arrest police said they believed he was responsible.
The gunman opened fire at the parade, near the city of Chicago, at around 10:15 local time (15:15 GMT), just a few minutes after it began.
The event was scheduled to include floats, marching bands, and community entertainment as part of the city's Independence Day celebrations.
But what should have been one of the happiest days of the year quickly turned to panic, with pushchairs, purses and lawn chairs left discarded on the street as crowds fled from the scene. Some witnesses said they thought the sound of gunfire was fireworks.
The gunman fired at members of the public from the rooftop of a nearby shop, where police recovered "evidence of a firearm." Read More...