A 21-year-old man found to be the assailant who went on a shooting spree during a July 4 parade in Highland Park, Illinois, has been charged with seven counts of first-degree murder on Tuesday, prosecutors said.

Robert Crimo III, the man allegedly responsible for seven deaths and more than 35 injuries, was disguised in women's clothing during the wealthy Chicago suburb massacre. He was arrested on Monday, shortly after the attack on the parade commemorating the Independence Day of the United States.

"There will be more charges," Lake County State's Attorney Eric Rinehart told reporters. "We anticipate dozens of more charges centered around each of the victims."

The initial death toll stood at six victims. However, police spokesperson Christopher Covelli said the toll rose to seven on Tuesday after one of those injured succumbed to their wounds and died in the hospital.

Covelli said no motive had been established for the attack that left a two-year-old child wandering alone after both his parents lost their lives during the shooting.

"We do believe Crimo pre-planned this attack for several weeks," and that he acted alone, he added.

The spokesperson also underlined that the authorities were yet to determine whether the attack was racially motivated or motivated by any other protected status, such as religion. He further revealed that Crimo had a history of mental health issues and threatening behavior.

Law enforcement officers had been called twice to the assailant's home in 2019, once to investigate a suicide attempt and a second because a relative said he had threatened to "kill everyone" in the family, Covelli explained.

Police found various melee weapons in his home, confiscating 16 knives, a dagger, and a sword, but no arrest was made.

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Crimo, as per Covelli, used a fire escape to gain access to the roof of a building overlooking the parade route and fired more than 70 rounds from a rifle "similar to an AR-15", which is just one of several guns that he had legally obtained.

The July 4 massacre is the latest in an epidemic of gun violence that has long been ravaging the United States, with authorities still debating stricter gun laws despite the latest spree of massacres. Around 40,000 deaths a year are caused by firearms in the US, according to the Gun Violence Archive.

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Two shootings in May that left 21 people dead, mostly young children, at an elementary school in Texas and 10 Black grocery patrons dead in upstate New York revived the US' bitter debate over gun regulation.

The recent spike in tragic shootings has pushed guns to the forefront of a national debate as US leaders grapple with how to reduce the alarming rate of violence.

The US Senate recently passed a bill aimed at curbing the gun violence ravaging the United States, which has been particularly rampant over the past couple of months.

The bill that passed the Senate floor includes narrow restrictions on firearm ownership and allocates several billion dollars to mental health and school security funding.

Biden called on members of Congress to pass tougher laws just a day after a mass shooting in Tulsa, Oklahoma, over a week after a school massacre in Uvalde, Texas, and almost three weeks after a mass shooting in Buffalo, New York City.

The Democrat called on lawmakers to raise - at a minimum - the age at which assault weapons can be purchased from 18 to 21. He highlighted the "unconscionable" fact that the majority of Senate Republicans do not want any of these proposals to be debated or come up for a vote.

Other measures include bolstered background checks, a ban on high-capacity magazines, maintaining safe storage of firearms, and allowing for the liability of gun manufacturers for crimes committed using their products.