Seven people were reportedly killed and more than 30 people were wounded at the Fourth of July parade in Highland Park, Illinois.
Authorities said the gunman shot more than 70 rounds into the crowd at random on Monday.
Children are among the wounded, but no child has died, authorities said.
Investigators believe the 21-year-old alleged shooter spent weeks pre-planning the attack.
"The rifle was purchased in Illinois and the information we have thus far is that it appears to have been purchased legally," said Deputy Chief Christopher Covelli of the Lake County Sheriff’s Office.
Covelli added that the gunman was wearing women's clothing at the time of the attack in an effort to conceal his identity. Following the shooting, Covelli said the shooter blended into the crowd and went to his mother's house.
He borrowed her car and was eventually pulled over and arrested.
"At this point, we have not developed a motive," Covelli said.
Disturbing videos of the alleged shooter have emerged online. Covelli said they were not aware of the videos prior to the attack, but they are looking into them.