The gunman in the deadly Independence Day attack in suburban Chicago fired more than 70 rounds at paradegoers and fled the scene by dressing as a woman to blend in with people frantically trying to escape the carnage, police said Tuesday.
Police said the shooter used an AR-15-style assault weapon during the attack that killed seven people and wounded more than 30 others in the affluent community of Highland Park during a Fourth of July celebration.
They said the shooter may have been planning the assault for weeks.
Police arrested a 21-year-old suspect, Robert Crimo III, hours later when a policeman spotted him driving in his car just outside Highland Park, a community of 30,000 people on the Lake Michigan shoreline.
Police allege he opened fire on parents and their children from the rooftop of a building adjoining the parade route as they watched marching bands and local dignitaries celebrate the country's 1776 founding.
Authorities have not filed criminal charges.
The gunfire was initially mistaken for fireworks celebrating the national holiday. Once the crowd realized it was gunfire, panicked paradegoers began to flee or scrambled into stores to escape the attack.
FBI agents and local police searched for more evidence Tuesday, sifting through trash cans along the parade route and looking under picnic blankets abandoned by families in their haste to escape. A day after the shooting, baby strollers, lawn chairs and other items left behind by panicked paradegoers remained inside a wide police perimeter.
Investigators who have questioned the suspect and reviewed his social media posts have not determined a motive for the attack, Lake County Major Crime Task Force spokesman Christopher Covelli told a news conference. Crimo legally bought the weapon found at the scene of the attack and a second rifle found in his car, Covelli said.
Police said they have not found any indication that the shooter targeted anyone by race, religion or other protected status.
Crimo, who goes by the name Bobby, is an aspiring rapper with the stage name Awake the Rapper, posting videos and songs, some ominous and violent, on social media sites.
The Highland Park attack was the latest mass shooting in the United States in recent weeks, with earlier assaults on people occurring at a grocery store in Buffalo, New York; an elementary school in Uvalde, Texas; and a medical office in Tulsa, Oklahoma.
U.S. President Joe Biden issued a statement saying he and his wife, first lady Jill Biden, were "shocked by the senseless gun violence that has yet again brought grief to an American community on this Independence Day."
The Highland Park shooting occurred more than a week after Biden signed the first major federal gun violence legislation passed by Congress in decades, although it would not have blocked the sale of the assault weapon used in Monday's attack.
The bipartisan compromise bill requires new background checks of gun buyers under 21 and provides more money to beef up security at schools and for mental health care for those considered at risk of harming themselves or others.
Some information for this report came from The Associated Press, Reuters and Agence France-Presse.