WAUKESHA, Wis. — It was supposed to be a celebratory night in Waukesha, Wis. Dance groups and high school bands and politicians were marching along Main Street in the Milwaukee suburb’s Christmas parade, which was returning from a pandemic hiatus.

Then, just before 4:40 p.m., the driver of a red S.U.V. stormed past barricades and barreled through the crowd, striking dozens. At least five people were killed and more than 40 people were injured, and the numbers could change, city authorities said in a statement late Sunday. Area hospitals reported treating dozens of patients, including many children. It was unclear what might have motivated the episode.