An Illinois man accused of shooting into a crowd watching a Chicago Independence Day parade was indicted by a grand jury on 117 counts, including 21 of first-degree murder, the state attorney's office said on Wednesday.
Robert Crimo has been held without bail since he was arrested after the shooting at the July Fourth celebration in Highland Park killed seven people and injured more than three dozen.
Mr Crimo is set to appear in court on August 3 for his arraignment, the Lake County State Attorney's office said.
Under the US legal system, a prosecutor can convene a panel of citizens, or a grand jury, that has the power to decide whether there is sufficient evidence to bring a defendant to trial.
If convicted on the murder charges, he would face a mandatory sentence of life in prison without the possibility of parole, Eric Reinhart, the state attorney for Lake County, said the day after the shooting.
The bloodshed was part of a recent flare-up of mass shootings that has renewed debate about gun violence in the US.
It followed an attack in Uvalde, Texas, in which 19 schoolchildren and two teachers were killed, and a shooting rampage at a supermarket in a predominately black neighbourhood of Buffalo, New York, in which 10 people were murdered.
Prosecutors said Mr Crimo had planned the attack for weeks before climbing to a rooftop and firing more than 70 rounds at parade spectators.
He then made his getaway dressed in women's clothing and make-up to cover his facial tattoos, they said.
The Smith and Wesson semi-automatic rifle, similar to an AR-15, used in the shooting was found at the scene.
The suspect had a similar weapon in his mother's car, which he was driving when he was arrested, county prosecutors said.
Police said they had no immediate evidence of any anti-Semitic or racist basis for the attack. The area has a large Jewish community.
Investigators were reviewing videos containing violent images, which Mr Crimo had posted on social media.
The Lake County Sheriff's office said he legally bought five guns, rifles and pistols, despite having come to law enforcement's attention two times earlier for alleged behaviour suggesting he might harm himself or others.