The man suspected of killing seven people and injuring dozens at an Independence Day parade in Chicago will face 117 charges.
Robert E Crimo III was arrested by police hours after shots were fired into the crowd during the Fourth of July celebrations in the Chicago suburb of Highland Park.
More than 70 rounds were aimed at partying families from a high-powered assault rifle.
The killer had dressed in women's clothing to blend in so they could escape the scene.
The state attorney's office has now confirmed Mr Crimo will be indicted on 117 counts relating to that day, including 21 of first-degree murder.
If convicted, he faces a mandatory life sentence without the possibility of parole.
The 21-year-old has been held without bail since he was arrested.
Prosecutors allege Mr Crimo planned the attack for weeks, before climbing to a rooftop and firing at parade spectators on 4 July.
The Smith & Wesson semi-automatic rifle, similar to an AR-15, used in the shooting was found at the scene.
It comes amid a number of mass shootings in the US that has renewed the debate about gun violence in the country.
A shooting attack in Uvalde, Texas, left 19 school children and two teachers dead, and a racially-motivated shooting rampage at a supermarket in Buffalo, New York, killed 10 people.
Investigators have reviewed a number of videos Mr Crimo had posted on social media apparently containing violent imagery.
A spokesperson for the local Lake County Sheriff's office said the suspect legally purchased five guns in all, rifles and pistols, despite having come to police attention on two previous occasions for alleged behaviour that suggested he might harm himself or others.
Shortly after the attack at the parade, a statement released via Twitter from Mr Crimo's parents said their hearts went out to the victims.
"We are all mothers and fathers, sisters and brothers, and this is a terrible tragedy for many families, the victims, the paradegoers, the community, and our own," they said. "Our hearts, thoughts, and prayers go out to everybody."