Five people have died and over 40 were injured after a car ploughed into a parade in Wisconsin, police say.
Footage posted online shows a red sports utility vehicle (SUV) driving through a Christmas parade in the city of Waukesha, west of Milwaukee, at around 16:40 local time (22:40 GMT), reports BBC.
Police chief Dan Thompson said it hit dozens of people including children.
One person is in custody. The incident does not appear to be an act of terrorism at this time, officials say.
The suspect appeared to have been fleeing another scene when he ran into people at the parade, a law enforcement official familiar with the early findings of the investigation told the BBC's US partner CBS News.
Local resident Angelito Tenorio told the Milwaukee Journal Sentinel newspaper he had just finished marching in the parade when the incident occurred.
"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," he said.
Corey Montiho said his daughter's dance team was hit by the SUV.
"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," he told the paper.
'Traumatic'
Chief Thompson said officers had recovered a suspect vehicle and a person of interest was in custody, but gave no further details. He added the investigation was "very fluid".
Police fired at the car to try to stop it, he said.
Fire chief Steven Howard told reporters that his department had taken 11 adults and 12 children to nearby hospitals after the incident. The city's police department later said Waukesha's public schools would be closed on Monday.
Mayor Shawn Reilly meanwhile said it was a "traumatic" time for the city. "My heart goes out to all those affected by this senseless act," he said.
One video published on social media shows the car crashing at high speed through street barriers. Another showed the vehicle driving into what looked like a group of musicians.
Local media reported that police officers ran down the street during the incident, telling people to take shelter in shops.
One witness said the driver also hit a group of "Dancing Grannies" and at least one person flipped over the front of the car.
The Milwaukee Dancing Grannies said some of its members "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 group wrote on its Facebook page.
Photos of a red SUV with a crumpled bonnet parked in a driveway were later posted on Twitter by TV station CBS 58.