Police said that at 4.39pm, the SUV plowed at high speed into band members and cheerleaders marching four abreast in close formation, hitting multiple people and scattering the crowd before driving on.
Dan Thompson, the local police chief, said his department had taken one person of interest into custody and seized the vehicle. He said he didn’t yet know if there was any connection to terrorism.
The City of Waukesha posted on its Twitter and Facebook accounts: “At this time, we can confirm that 5 people are deceased and over 40 are injured. However, these numbers may change as we collect additional information.”
Earlier, the officials had said that 11 adults and 12 children were taken to local hospitals. The Children’s Wisconsin hospital said it had received 15 patients and that there were no fatalities among those.
Video from the city’s parade live stream, as well as phone clips from attendees, showed a red SUV breaking through barriers and speeding into the roadway where the parade was taking place.
Shawn Reilly, the Waukesha mayor, called the incident a “horrible and senseless act.”
Mr Thomson also called it a “very tragic incident, very chaotic.” He said that during the incident, “an officer discharged his weapon at the suspect vehicle to try to stop the vehicle.” He added that no bystanders were injured as a result of weapon discharge.
A local, Angelito Tenorio, told the Milwaukee Journal Sentinel: “We saw an SUV... just put the pedal to the metal and just zooming full speed along the parade route. And then we heard a loud bang, and just deafening cries and screams from people who are struck by the vehicle.”
Another witness named Corey Montiho, whose daughter was hit by the SUV, told the newspaper that “there were pom-poms and shoes and spilled hot chocolate everywhere. I had to go from one crumpled body to the other to find my daughter”.
Mr Montiho added: “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.”
Tom Hickey, 25, who marched at the rear of the parade, said he saw the red Ford Escape slowly making its way through the crowd and honking for people to get out of the way.
Mr Hickey said he thought the driver was lost, but then saw him turn right onto the parade route, gun the engine and accelerate into the crowd.
Mayor Reilly said: “Today our community faced horror and tragedy in what should have been a community celebration. My heart goes out to all those affected by this senseless act.”
Tony Evers, the Wisconsin governor, said that he was praying for the city of Waukesha.
As well as young children taking part in the parade, videos and eyewitness reports showed that the group known as the “Milwaukee Dancing Grannies” was also struck by the SUV. All members are in their 50s to mid-70s, local reports said.
The group on their Facebook page said: “Members of the group and volunteers were impacted and we are waiting for word on their conditions. Please keep the Grannies, all those injured, and all those who witnessed this horrible event in your thoughts and prayers.”
The Catholic Community of Waukesha also wrote on Facebook that several parishioners were injured and were being hospitalised. The Sentinal reported that the Archdiocese of Milwaukee confirmed that one of those injured was a Catholic priest “as well as multiple parishioners and Waukesha Catholic School children.”
Police sent an emergency alert to people’s smartphones after the incident, hoping to keep the scene clear while they investigated.
Nonetheless, it was reported that after the police shut down several streets in downtown Waukesha, many parade-goers were still seen lingering in the area as they came to terms with the tragedy that had unfolded.
More than a dozen units from both fire department and police department were on the scene, with the FBI and the ATF (the federal Bureau of Alcohol, Tobacco, Firearms and Explosives) assisting Waukesha police.
A family reunification location was set up at the Metro Transit Center by Bank Street, according to the police department’s Facebook page.
Democrat Sara Rodriguez, who represents parts of Waukesha, and was marching in the parade with the Waukesha County Democrats. She told reporters that “I didn’t directly witness the car coming through — we had finished the parade route and so it came after when we were done.” She added: “People were running and screaming and moving out of the way. … My thoughts and prayers are with the families of those who’ve been injured.”