Shortly before midnight, the City of Waukesha posted revised casualty totals to its Twitter and Facebook accounts.
"At this time, we can confirm that 5 people are deceased and over 40 are injured," the statement said. "However, these numbers may change as we collect additional information."
Earlier, officials said 11 adults and 12 children were transferred to local hospitals. Friends and family took the others. Children's Wisconsin hospital said it had 15 patients and no deaths.
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Waukesha Mayor Shawn Reilly called the incident a "horrible and senseless act."
Waukesha Police Chief Dan Thompson said that around 4:39 p.m., a red SUV broke through the parade barriers and headed west on Main Street.
"The vehicle struck more than 20 individuals. Some of the individuals were children and there were some fatalities as a result of this incident," he said.
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A suspect vehicle was found, and a "person of interest" was arrested, the police chief said. People were admitted to hospitals via ambulances and police vehicles; he continued.
"Very tragic incident, very chaotic," Thompson said.
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Thompson said during the incident, "an officer discharged his weapon at the suspect vehicle to try to stop the vehicle. No bystanders were injured as a result of the weapon discharge."
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The incident happened during one of the city's most notable and cherished annual events. A joyous group of people lined both sides of the road, people bundled against the cold.
One video showed the red SUV on the parade route, turning down the street and pulling left behind the Waukesha South High School Black Shirt Band.
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Corey Montiho, a School District of Waukesha board member, was by Mainstream Bar & Grill when he heard that his daughter's youth dance team was hit.
"There were pom-poms and shoes and spilled hot chocolate everywhere," he said. "I had to go from one crumpled body to the other to find my daughter. My wife and two daughters were almost hit. Please pray for everybody. Please pray. My family is safe but many are not. I held one little girl's head in my hand, she was seizing and she was bleeding out of her ears. I held her mother as she collapsed. Please pray."
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In a Facebook post, the Waukesha Xtreme Dance team thanked those who checked on the safety of team members and their families.
"Please keep them in your thoughts and prayers," the group said, suggesting it would provide "an update when we know more."