Robert Crimo, 22, was identified as a "person of interest" after a deadly sniping attack in the Chicago suburb of Highland Park, Illinois, where a rooftop gunman with a high-powered rifle turned an Independence Day parade celebration into a scene of death and trauma.
Police officials said the shooting began at 10:14 am, when the parade was approximately three-quarters of the way through.
"It sounds like spectators were targeted... So, very random, very intentional and very sad," said Lake County Major Crime Task Force spokesman Christopher Covelli.
Five of the six people killed, all adults, died at the scene. The sixth was taken to the hospital but succumbed to wounds there.
Firing into the holiday crowd, the shooter caused scenes of chaos as panicked onlookers ran for their lives, leaving behind a parade route strewn with chairs, abandoned balloons and personal belongings.
Emergency officials said around two dozen people, including children, were treated for gunshot injuries, with some in critical condition.
After a brief car chase, Crimo was taken into custody "without incident," Highland Park police chief Lou Jogmen told reporters.
Emily Prazak, who marched in the parade, described the mayhem.
"We were getting ready to march down the street and then all the sudden waves of these people started running after, like running towards us. And right before that happened, we heard the pop, pop, pop, pop, pop, and I thought it was fireworks," Prazak told AFP.
She added: "This is the day that we celebrate our country. This is also a day that our freedom got stolen from us -- because many of us residents here, in this building even, we're all locked down."
The shooting is part of a wave of gun violence plaguing the United States, where approximately 40,000 deaths a year are caused by firearms, according to the Gun Violence Archive website.
And it cast a pall over America's Independence Day, in which towns and cities across the country hold similar parades and people -- many dressed in variations on the US flag -- hold barbecues, attend sports events and gather for firework displays.
President Joe Biden voiced his shock and vowed to keep fighting "the epidemic of gun violence" sweeping the country.
"I'm not going to give up," he said.
Last week, Biden signed the first significant federal bill on gun safety in decades, just days after the Supreme Court ruled that Americans have a fundamental right to carry a handgun in public.