The tragedy, in a country still reeling from a recent series of deadly shootings, took place in Highland Park, in the north of the United States.
A few hours later, the police arrested the suspect in the shooting that left at least six people dead and 26 injured on Monday during a parade near Chicago for the American national holiday on July 4.
Hundreds of police searched for the young man who, armed with a powerful rifle, had fired on the crowd gathered in this wealthy city of 30,000 inhabitants, located on the shores of Lake Michigan.
The man had posted himself on the roof of a business, sowing panic before fleeing.
The police had identified a suspect, Robert - known as "Bobby" - Crimo, 22 years old and "from the region".
She had released the photo of a diaphanous young man, with an emaciated and tattooed face.
The photo of the suspect released by the authorities.
Government/Handout via Reuters
"Be very vigilant, this person is on the run," warned one of his spokespersons, Christopher Covelli.
"She is considered armed and very dangerous."
After a brief car chase, the suspect was arrested "without incident", Highland Park Police Chief Lou Jogmen told reporters Monday evening.
The man has several pages on the internet, where he is presented as a musician from Chicago nicknamed "Awake the rapper".
The authorities had deployed a huge security device with armored vehicles and federal reinforcements.
" Battlefield "
On this holiday Monday, hundreds of people, including many families with children, took their places on the sidewalks of Highland Park to attend, as in all the United States, the traditional American Independence Day parade.
As high school marching bands marched through the streets, a flurry of gunfire rang out.
"We heard about 50 gunshots, we all thought it was fireworks," a woman who witnessed the tragedy, who only gave her first name, Zoe, commented on CNN.
"At one point, I saw a girl covered in blood, I had never seen that," she added, describing a "panic, with people falling one after the other."
She hid behind a trash can before being sheltered in a basement.
On leaving, she discovered “a battlefield”.
According to the first elements of the investigation, the shooter fired from the roof of a business, accessible by an emergency staircase.
“He was very discreet and difficult to see,” Mr. Covelli described.
He immediately fled but a "powerful" rifle was found by the police.
Five people, all adults, died on the spot and a sixth after being transferred to hospital.
At least 26 injured, aged 8 to 85, were taken care of by the emergency services and some, including at least one child, were in critical condition, according to a hospital official.
The festivities were immediately suspended in several surrounding towns, which also closed their beaches and called on the population to be extremely careful.
"The Epidemic of Gun Violence"
The tragedy immediately revived the debate on firearms, of which nearly 400 million are in circulation in the United States.
"It's damning that a party in honor of America is sabotaged by a quintessentially American scourge," said Illinois Democratic Governor JB Pritzker.
“The only freedom we deny our citizens is to live without the daily fear of guns.
Saying he was "shocked", Democratic President Joe Biden for his part promised in a press release to "not give up the fight against the epidemic of gun violence" after this new bloodbath.
The United States is still reeling from a series of shootings, including one in an elementary school in Uvalde, Texas, which killed 21 people, including 19 children on May 24.
The country is more generally facing an increase in gun violence, with more than 22,000 people killed since the start of the year, according to the Gun Violence Archive, which includes suicides in its data.