0327 GMT November 29, 2021
Waukesha Police Chief Dan Thompson told reporters that one person was in custody and a vehicle had been recovered after the incident in the town of around 72,000 people, 20 miles (32km) west of Milwaukee, Reuters reported.
Thompson said it was not clear whether the incident was terrorist in nature.
A video posted online showed a red sport utility vehicle driving at speed alongside and into the parade, appearing to run over more than a dozen people before crowds ran from sidewalks to offer assistance.
Thompson said an officer fired shots at the vehicle in an attempt to stop the carnage. Another video on social media appeared to show police firing on the vehicle as it crashed through street barriers.
Nurse Jodi Singsime, 42, took shelter in a store with members of her family before going out to help the injured.
She choked up as she described coming across a small boy in the road: "I felt his neck for a pulse and he had one but his eyes were barely open and his face – all I can remember is his sweet little innocent face was purple."
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.
White House spokeswoman Jen Psaki said President Joe Biden had been briefed on the "tragic situation" and federal officials were in contact with local authorities to offer assistance.
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, which occurred shortly after 4:30 p.m. (2230 GMT). People bundled up against the cold huddled on sidewalks as dusk fell.
The Children's Wisconsin hospital said it had received 15 patients as of 8 p.m., with no reported fatalities at that time.
Schools will remain closed on Monday for grades 4K-12 and additional counselors will be available for students, the district superintendent of schools said.
In 2015, four people were killed and 46 injured in Stillwater, Oklahoma, when a woman drove into a crowd watching the homecoming parade for a football team.