By Eric Beech and Alexandra Ulmer
Waukesha Police Chief Dan Thompson said a person of interest was in custody and the suspect vehicle had been recovered after the incident in Waukesha, about 20 miles (32 km) west of Milwaukee.
A shelter-in-place order had been lifted, police said.
“A red SUV drove into our Christmas parade that we were holding downtown. More than 20 individuals were injured as a result of this incident,” Thompson told reporters.
Asked about the fatalities, Thompson said: “I don’t have an exact number at this time.”
It was not known whether the incident was related to terrorism, he added.
A video posted online of the incident showed a red SUV plowing through the parade, appearing to run over more than a dozen people.
In a second video, police appeared to open fire on the vehicle as it crashed through street barriers.
The local CBS affiliate later showed a picture on Twitter of what appeared to be the red SUV involved with its hood crumpled and front fender hanging off, parked in a driveway.
A woman told Milwaukee’s Fox6 TV station that the SUV hit a dance team of girls between 9 and 15 years old. She said the immediate reaction was silence, followed by screaming, running and checking on those injured, Fox6 reported.
One witness said the driver hit the “Dancing Grannies” and at least one person flipped over the hood of the SUV, according to Milwaukee’s WISN-TV, an ABC affiliate.
Another witness estimated the SUV was going about 40 mph (64 kph) when it hit the crowd, the TV station said.
Photos and videos of Waukesha circulating on Twitter showed police cars and ambulances crowding a street decked out with Christmas lights in the aftermath of the incident. Bundled up people huddled on sidewalks as dusk fell.
Some of the injured were taken to the hospital by ambulance, others by police and still more by family and friends, Thompson said.
The incident occurred around 4:39 p.m. (2239 GMT), police said.
“As we were walking back in between the buildings … we 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,” Angelito Tenorio, an alderman in nearby West Allis, told the Milwaukee Journal Sentinel.
Police officers ran down the street, telling parade watchers to take shelter in stores, the newspaper said. (Reporting by Eric Beech and Alexandra Ulmer; Writing by Lincoln Feast; Editing by Noeleen Walder, Himani Sarkar and Peter Cooney)