Mr Crimo was taken into custody on Monday night after a brief police pursuit near Lake Forest, a city north of Chicago.
He had been the subject of an intensive manhunt after earlier being identified as a suspect in the shooting.
Prosecutors said the shooter took up a sniper position on a rooftop before opening fire and triggering scenes of chaos as people ran for cover to save their lives.
Chris Covelli, of the Lake County Sheriff’s Office, revealed the suspect had “planned his attack for several weeks” and dressed in women’s clothes to conceal his identity during the shooting.
He accessed the roof via a fire escape and used a Smith & Wesson M&P 15 rifle to fire more than 80 rounds into the crowd.
After the shooting, he was able to come down the fire escape and melt into the crowd before walking to his mother’s home, who lived nearby, and taking her car.
A member of the public identified Mr Crimo’s vehicle from information issued by authorities and called 911, and he was arrested shortly afterwards after police located the car.
A search of his home also uncovered multiple other firearms including pistols.
The motive for the attack is still unknown, but police said they did not believe it was racially motivated.
Five people died at the scene. Two more later died in hospital.
Mr Crimo was charged with seven counts of first-degree murder on Tuesday night. Prosecutors said further charges would be laid against him.
Who is Robert Crimo?
Mr Crimo goes by the nickname Bobby and is an amateur rapper. He had allegedly uploaded a number of videos online, many months before the shooting, filled with violent imagery and depictions of mass murder.
He goes by the stage name “Awake the Rapper” and has more than 16,000 listeners per month on his Spotify page.
His social media footprint indicates that he is a video-game enthusiast who appeared to fantasise about mass shootings.
A recent video purportedly posted by Mr Crimo on YouTube appeared to show the aftermath of a school shooting with him draping himself in an American flag.
In another animated video titled “Toy Soldier”, he appeared to show his own demise.
The video showed a man, seemingly personifying Mr Crimo, wearing a black T-shirt with his YouTube channel’s logo on it and lying in a pool of blood during a confrontation with the police.
The video begins by showing a student texting in class while Mr Crimo raps “f*** this world” with visuals showing a heavily armed shooter opening fire inside a school.
In another video, in which he appears in a classroom wearing a black bicycle helmet, he says he is “like a sleepwalker. I know what I have to do”, adding that “everything has led up to this. Nothing can stop me, even myself”.
The YouTube channel that posted the videos was made unavailable on Monday night after having been accessible to the public for months.
In 2020, he was pictured attending a rally of Donald Trump dressed as the main character from “Where’s Waldo?” In another photo on Twitter, he was seen wrapped in a Trump flag like a cape. He was also pictured sporting an FBI cap and he had liked a video of Joe Biden.
Mr Crimo had his own Discord server named “SS”, where he would chat with people. It has since been taken down. According to NBC news, the community featured a politics board with several nihilistic memes.
The most recent post on the community before the shooting was in March, showing a picture of Budd Dwyer, the Pennsylvania state treasurer who shot himself on live television in the late 1980s with the caption, “I wish politicians still gave speeches like this”.
Discord users shared pictures of Mr Crimo with one selfie seemingly making a reference to suicide. Its caption read: “Cursed image screenshot and send to everyone or commit not alive anymore (sic)”.
On Tuesday, Spotify and Youtube removed all of Mr Crimo’s content, according to CNN.
According to the FBI, Mr Crimo has tattoos on his face showing four tally marks with a line striking through them on his right cheek and a cursive script above his left eyebrow that reads “Awake”.
Mr Crimo also has tattoos of red roses and green leaves on his neck.
Suspect threatened to ‘kill everyone’
Mr Crimo threatened to “kill everyone” in his home with a “collection of knives” three years before he allegedly went on his shooting rampage.
Police said in a press conference on Tuesday afternoon that officers were called to his home for incidents twice in 2019.
The second time – in September 2019 – police were called to a report from a family member that Mr Crimo had vowed to “kill” the people inside the home and that he had a “collection of knives”, which included a sword.
Officers responded to the scene and confiscated the knives but did not arrest the suspect.
This came after police were first called to the home in April 2019 to a report that Mr Crimo had tried to kill himself.
Despite the incidents, Mr Crimo was able to legally purchase multiple firearms in the lead-up to Monday’s attack.
A bizarre obsession with the number 47
The 21-year-old appears to have an obsession with the number 47.
Police believe Robert “Bobby” Crimo III dressed in a wig and women’s clothes before using a high-powered rifle to fire more than 70 rounds into a crowd in the Chicago suburb on Monday morning, killing six and wounding dozens.
Mr Crimo’s extensive digital footprin frequently referenced the number 47.
He also had the number tattooed on the side of his face, and it was also painted on the door of a car parked outside his family home.
First court appearance
Mr Crimo was charged with seven counts of first-degree murder by prosecutors from the Lake County State’s Attorney’s Office on Tuesday evening.
He appeared in court for the first time on Wednesday morning where he was ordered to be held without bond on seven counts of first-degree murder.
Dressed in a black t-shirt and his long dark hair down around his face, he stared expressionless into the camera.
The judge ordered that he be held without bond. A public defender has been appointed to defend him.
In a press conference outside the court on Wednesday, Mr Covelli said that the suspect crossed state lines from Illinois to Wisconsin, driving all the way to Madison in his mother’s vehicle.
“Investigators did develop some information that it appears when he drove to Madison, he was driving around, however, he did see a celebration that was occurring in Madison, and he seriously contemplated using the firearm he had in his vehicle to commit another shooting in Madison,” he said.
Robert Crimo’s family
Mr Crimo’s father has been identified as Bob Crimo Sr, 58, a local deli owner who once ran for mayor of Highland Park in 2019 but had lost by a two-to-one margin to the incumbent.
Just days after the Uvalde elementary school shooting in Texas in which 19 children and two teachers were killed in a mass shooting, a Twitter account that purpotedly belongs to Mr Crimo Sr liked a tweet in support of the second amendment.
“Protect the Second Amendment like your life depends on it,” read the 28 May tweet.
His mother Denise Pesina was seen angrily confronting a SWAT team in the aftermath of the July 4th mass shooting.
Video obtained by CBS News shows Ms Pesina shouting and gesturing at officers in McDaniels Avenue, Highland Park, on Monday, as a massive manhunt was underway for her son.
It’s unclear what the confrontation was about, but the network reported that Ms Pesina was in contact with police over for several hours after the Independence Day parade shooting.
It has also emerged that a business owned by Ms Pesina had a mailbox at a UPS Store just a few blocks from where Monday’s shooting was carried out.
Mr Crimo was the middle child of three siblings in an Italian-American family in Highland Park, Illinois.
His uncle, Paul Crimo, told Fox 23 Chicago that the family saw no “warning signs” that would indicate he could possibly be involved in the shooting.
“I saw no signs of trouble. And if I did see signs I would have said something,” his uncle said. He said that his nephew was a “YouTube rapper” who once worked at the Panera Bread bakery company.