"At this time, we can confirm that five people are deceased and over 40 are injured," the Waukesha Police Department said on its Facebook page. "However, these numbers may change as we collect additional information."
Waukesha Police chief Dan Thompson earlier told reporters that a person of interest was in custody and a vehicle had been recovered after the incident in the town, about 32 km west of Milwaukee.
It was not known whether the incident was related to terrorism, but there was no further danger and an earlier shelter-in-place order in the town of around 72,000 had been lifted, Thompson said.
A video posted online showed a red sport utility vehicle plowing at speed through 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. Police did not believe shots had been fired from the vehicle, Thompson added, contrary to earlier reports.
Belen Santamaria, her husband, and their three-year old daughter were planning to join the parade with their Catholic Church. But Santamaria, a 39-year-old Mexican factory worker, woke up on Sunday with a backache, so the family watched the procession from the sidewalk instead.
"The SUV came by at full speed," said Santamaria. "Then I started to hear people screaming."
She hid in a restaurant with their daughter while her husband, 39-year-old delivery worker Jesus Ochoa, ran ahead to try to help injured people. He said he heard through other members of their church that roughly 10 members of the congregation, mostly Latinos and both adults and children, are injured.
"I was going to be there, walking," Santamaria said. "The SUV would have hit us too."