A day of national celebration turned to tragedy Monday when a gunman killed six people and injured dozens more at a July Fourth parade in Highland Park, Illinois -- leaving the nation grieving yet another mass shooting.

The suspected shooter, identified by authorities as Robert E. Crimo III, used a "high-powered rifle" in an attack that appeared to be "random" and "intentional," police said. They believe the shooter, who was apprehended later Monday, climbed onto a rooftop of a business and opened fire on the parade about 20 minutes after it started.

Some bystanders initially thought the sound of gunfire that pierced the sunny parade just after 10 a.m. CT along the town's Central Avenue, about 25 miles north of Chicago, was fireworks, until hundreds of attendees started to flee in terror -- abandoning strollers, chairs and American-flag paraphernalia on the streets.

Eyewitnesses described grabbing their children and families and running for their lives, some hiding behind dumpsters or in nearby stores for safety amid the chaos. One paradegoer described seeing a girl shot and killed, another saw a man shot in the ear with blood all over his face.

"It looked like a battle zone, and it's disgusting. It's really disgusting," Zoe Pawelczak, who attended the Independence Day parade with her father, recalled Monday.