Police have said there were "some fatalities" after a car ploughed into a parade in Wisconsin.
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).
Police chief Dan Thompson said it hit more than 20 people including children.
He confirmed people had died but said they would not give details until family members had been told.
Mr 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" and the injured has been taken to hospital by ambulance, police and family members.
Police fired at the car to try to stop it, he said. The FBI is assisting with the investigation.
Mayor Shawn Reilly said it was a "traumatic" time for the city. "My heart goes out to all those affected by this senseless act," he told a press conference.
One video shows the car crashing at high speed through street barriers. Another showed the vehicle driving into what looked like a group of musicians.
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.
Police officers ran down the street, telling people to take shelter in shops, local media reported.
One woman told TV station Fox6 the car hit a dance group of girls aged between nine and 15 years old.
The White House is "closely monitoring" the situation in Waukesha, an official said, and is offering support "as needed" to state and local officials.