A vehicle that sped into a Christmas parade in the US state of Wisconsin on Sunday caused five deaths and left more than 40 people injured, police said. 

Police have arrested a "person of interest" in connection with the attack in the city of Waukesha, which is within the Milwaukee metropolitan area. 

What do we know so far?

Live videos of the parade captured the moment the car broke through a barrier just after 4:30 p.m. local time (2230 UTC) and sped towards the parade at about 40 mph (64 kph), according to a witness.

"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.

"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.

In another video seen online the car plows into a marching band, leaving injured people in its wake.

A police officer had fired at the car to get it to stop, authorities reported.

Authorities said 11 adults and 12 children were taken to hospital by the emergency services alone. He added that police had arrested a suspect.

Thompson said it was unclear whether the incident was a terrorist attack.

"Waukesha police department has recovered a suspect vehicle," Thompson said. "It's an ongoing investigation." 

The FBI is also helping local law enforcement with the investigation. 

Victims rushed to hospital

Thompson said victims were transported to the hospital by ambulances, police cars and family members.

"There were small children laying all over the road, there were police officers and EMTs doing CPR on multiple members of the parade," local businessman Chris Germain said. He had come to the parade with 70 girls from his dance studio.

Local official Angelito Tenorio said he was at the parade when the vehicle sped into the area.

"We heard a loud bang, and just deafening cries and screams from people who were struck by the vehicle," Tenorio said. "Then we saw people running away or crying, and there were people on the ground who looked like they'd been hit by the vehicle."

"They were pompoms and shoes and spilled hot chocolate everywhere. I had to go from one crumpled body to the other to find my daughter," Corey Montiho, a Waukesha school district board member, told the Milwaukee Journal Sentinel.

The Children's Wisconsin hospital said it was treating 15 patients as of 8 p.m., with no reported fatalities. Other victims were distributed between six other hospitals in the area. 

"It was a very tragic incident, very chaotic. There are no other threats involved. The scene is now safe," said Thompson.

The White House was monitoring the incident and briefing US president Joe Biden. It is not known yet if the attack was linked to terrorism.

Schools would not open for many children in the city, with extra counselors brought in to support students struggling with the events, the district superintendent of schools said. 

The attack occurred just 55 miles (88 kilometers) north of Kenosha where Kyle Rittenhouse was freed on Friday after killing two men.

Wisconsin reacts to 'senseless act' 

Top Wisconsin officials reacted in anger and sadness to the attack that rocked the city of just over 70,000 people. 

"What took place in Waukesha today is sickening, and I have every confidence that those responsible will be brought to justice,'' tweeted the state's attorney general Josh Kaul.

"Kathy and I are praying for Waukesha tonight and all the kids, families, and community members affected by this senseless act," Wisconsin Governor Tony Evers said on Twitter.

"I'm grateful for the first responders and folks who acted quickly to help, and we are in contact with local partners as we await more information," he added.

Wisconsin US Senator Ron Johnson echoed Evers in a tweet, adding: "My gratitude for all law enforcement, medical personnel, and community members who are responding."

"The horrific violence at the Waukesha Christmas Parade is just heartbreaking," tweeted Tammy Baldwin, another Wisconsin senator.

"Please keep the victims in your prayers tonight."

ar, jc/wd (Reuters, AP)