The man accused of the July 4 mass shooting, which left at least seven dead and dozens wounded in Chicago, planned the attack for weeks and wore women's clothing to blend into the fleeing crowd, officials said on Tuesday.
Robert Crimo, 21, was charged on Tuesday with seven counts of first-degree murder.
If convicted, he could face a maximum sentence of life in prison without the possibility of parole, Illinois State Attorney Eric Reinhard said in announcing the charges.
Mr Reinhard said the murder charges would be followed by dozens of others before the investigation was over.
He said he would ask that the accused be held in custody without bail at his first court appearance on Wednesday.
Authorities said Mr Crimo fired more than 70 bullets into the crowd at an Independence Day parade in the affluent suburb of Highland Park, about 40 kilometres north of Chicago.
They said he used a “high-powered” rifle that he had bought legally.
“Following the attack, Crimo exited the roof, he dropped his rifle and he blended in with the crowd and he escaped,” said Chris Covelli, a spokesman for the Lake County Sheriff’s Department.
“Crimo was dressed in women's clothing and investigators believe he did this to conceal his facial tattoos and his identity, and help him during the escape with the other people who were fleeing the chaos.”
Mr Crimo was arrested on Monday evening after surrendering to police.
Police said they did not yet know the motive behind the shooting but were investigating alarming posts and videos Mr Crimo made.
“I think at some point, this nation needs to have a conversation about these weekly events involving the murder of dozens of people with legally obtained guns,” Highland Park Mayor Nancy Rotering told NBC.
Monday's shooting was the latest to rock the US.
A string of mass shootings included a racist attack at a supermarket in Buffalo, New York, which killed 10 people, and one in a primary school in Uvalde, Texas, in which 19 children and two teachers were killed.
“This tragedy never should have arrived on our doorsteps and as a small town, everybody knows somebody who was affected by this directly and, of course, we're all still reeling,” Ms Rotering said.
She said she knew the accused when he was a member of the Cub Scouts as a young boy. She was one of the local group's leaders.
Nicolas Toledo, who had travelled from Mexico to visit his family, was the first victim to be identified in the latest mass shooting.
His granddaughter, Xochil Toledo, described him as "a loving man, creative, adventurous and funny".
"As a family, we are broken and numb,” Ms Toledo said.
Toledo had not wanted to attend the July 4 parade in Highland Park, but his disabilities meant he was not to be left alone.
The father of eight was sitting in his wheelchair along the parade route when the shooting began. Three bullets struck him, killing him at the scene.
Ms Toledo said her father was shot in the arm while trying to shield her grandfather from the bullets. Her boyfriend and father suffered non-life-threatening injuries.
A GoFundMe page to send Toledo's body back to Mexico had raised more than $36,000 as of Tuesday morning.
He spent most of his life in Mexico and had been visiting his family in Illinois for the past month.
US President Joe Biden tweeted that he and first lady Jill Biden were shocked by the “senseless gun violence” tearing at America.
Another victim identified from Monday's mass shooting was Jacki Sundheim, a teacher at a synagogue in Highland Park.
Sundheim was remembered as a lifelong member of the North Shore Congregation Israel and a “cherished member” of its staff.
“There are no words sufficient to express the depth of our grief for Jacki’s death and sympathy for her family and loved ones,” the synagogue said on its website.
“We know you join us in the deepest prayer that Jacki’s soul will be bound up in the shelter of God’s wings and her family will somehow find comfort and consolation amid this boundless grief.”
Police confirmed that a third person had succumbed to their wounds after the shooting.
Steve Straus, 88, was a financial adviser, and his family told The New York Times that he had taken the train into Chicago to work from his office every day.