Five people were killed and 40 others wounded after a vehicle plowed into a Christmas parade in the US state of Wisconsin on Sunday, police said.

"We can confirm that 5 people are deceased and 40 are injured. However, these numbers may change as we collect additional information," the Waukesha Police Department said in a statement on their official Facebook page, adding a "person of interest" was in custody.

Officials were still gathering information on the incident which occurred just after 4:30 pm (2230 GMT), as spectators in the town of Waukesha, a Milwaukee suburb, watched the annual tradition.

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"The Waukesha Christmas parade was taking place when a red SUV broke through the barricades, westbound, headed down Main Street," Police Chief Dan Thompson told reporters earlier.

"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. He did not specify the number of people killed and added that no further information would be released until families have been notified.

A total of 11 adults and 12 children were taken to six area hospitals, Fire Chief Steven Howard told reporters.

A "person of interest" was in custody, Thompson said, adding that officers had recovered the involved vehicle. There were no other threats, officials at the press conference said.

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During the incident an officer did fire at the SUV in an attempt to stop it, the authorities added.

Schools will not open Monday and roads will remain closed, Thompson said, while the investigation is continuing. 

- Deafening cries -

While local law enforcement were the primary responding entities, the FBI said it was providing assistance.

Angelito Tenorio, who is running for Wisconsin state treasurer, was at the parade and told the Milwaukee Journal Sentinel that he "saw an SUV cross over, 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," he said.

According to bystanders and footage, the SUV sped into the parade behind a school marching band.

"All I heard was screaming and then people yelling out their children's names. That's all I heard" another witness, Angela O'Boyle, whose apartment overlooked the parade, told CNN.

Wisconsin Governor Tony Evers said he and his wife were "praying for Waukesha tonight and all the kids, families, and community members affected by this senseless act."

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Various lawmakers expressed their condolences, including the state's two senators, with Democrat Tammy Baldwin writing on Twitter that she found the "horrific violence... just heartbreaking." 

Her Republican counterpart Ron Johnson offered his "prayers for everyone injured" and "gratitude for all law enforcement, medical personnel, and community members who are responding."

Wisconsin Governor Tony Evers said he and his wife were "praying for Waukesha tonight and all the kids, families, and community members affected by this senseless act."

The crash comes after a tumultuous week in the state of Wisconsin following the conclusion of the trial of Kyle Rittenhouse, an American teen acquitted after fatally shooting two men during protests and riots against police brutality last year in Kenosha, located just 50 miles (80 kilometers) from Waukesha.