A prosecutor broke the news at the first court appearance by suspect Darrell Brooks.
He was charged with five counts of intentional homicide as he appeared in court over the Waukesha Christmas parade attack and if convicted he would face a mandatory life sentence in prison.
Prosecutors said they now plan on filing an additional count of intentional homicide later this week and could bring lesser charges in relation to those people injured during the incident.
The sixth victim has been named on a GoFundMe page as Jackson Sparks, aged eight.
Both he and his brother Tucker, 12, were injured when the SUV driven by Brooks entered the parade route.
Jackson underwent brain surgery on Sunday evening but succumbed to his injuries and passed away on Tuesday afternoon.
Tucker sustained road rash and a fractured skull but is recovering from his injuries and has been discharged.
His parents Aaron and Sheri have asked for privacy to allow for Tucker’s recovery and for time to mourn the loss of Jackson.
The eight-year-old became the sixth victim of the Wisconsin Christmas parade incident last Sunday.
“Aaron Sparks and Sheri Sparks would like to thank everyone for your prayers and amazing support during this difficult time,” said LifePoint Church in a Facebook post.
“They wanted to give an update to everyone as you are likely aware, Tucker (12) and Jackson (8) Sparks were both seriously injured during the Waukesha Christmas Parade attack and have been in the ICU since.
“Tucker, by the grace of God is miraculously recovering from his injuries and will be being discharged home. This afternoon, our dear Jackson has sadly succumbed to his injuries and passed away.
“Please know that they appreciate your continued prayers and tremendous outpouring of support for their family. They do however ask for privacy at this time to allow Tucker to continue to heal physically and their family to heal and mourn the tremendous loss of their sweet little boy who is now under the care of Jesus.”
In addition to the six deaths, 62 people were injured in the chaos that unfolded in the Wisconsin city, about 32km (20 miles) west of downtown Milwaukee.
The defendant appeared to be wearing a protective vest as he was led into court to sit beside his lawyers, and prosecutors asked for $5m cash bail saying “it is warranted given his history” and described him as “a threat to our community”.
Commissioner Kevin Costello said that he believed Mr Brooks was a flight risk and that he didn’t “do well under pressure”.
“You don’t respond well to common sense within our community, our society,” he said as he set bail at $5m.
“I am an old guy who has been doing this for almost 40 years. The nature of this offence is shocking, the detail of which I was not expecting here today,” said Commissioner Costello.
“Two detectives not only tried to stop this, but rendered an opinion that this was an intentional act. You are presumed innocent sir, but that is what the allegations are. I have not seen anything like this in my entire career and it seems like a very strong case for the state.
“The bail is extraordinarily high but it is an extraordinarily big case and there is an extraordinary history of this gentleman hurting people and not following court orders, not following criminal laws or societal norms.”
The other five deceased victims have been identified as Virginia Sorenson, 79; LeAnna Owen, 71; Tamara Durand, 52; Jane Kulich, 52; and Wilhelm Hospel, 81.
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