Authorities have so far confirmed the deaths of five people after an SUV broke through a roadblock and plowed into a Christmas parade.
More than 40 people were injured, about a dozen of whom are believed to be children taking part in the annual parade.
The White House said on Sunday night that it was monitoring the situation in Wisconsin closely and that US President Joe Biden had also been briefed on the tragedy.
“The White House is closely monitoring the situation in Waukesha and our hearts go out to everyone who has been impacted by this terrible incident,” the statement read, as ABC News reported. “We have reached out to state and local officials to offer any support and assistance as needed.”
White House press secretary Jen Psaki also tweeted that Mr Biden would receive regular updates and that: “Our hearts are with the families and the entire community.”
Wisconsin governor Tony Evers meanwhile described the tragedy as a “senseless act” in a statement on Sunday night and said: “I’m grateful for the first responders and folks who acted quickly”.
The city of Waukesha tweeted that many of the injured had transported themselves to hospital following the attack, and that the “scene is still fluid”.
“The community mourns the loss of our community members and those injured. We are asking for the community to come together during this time and to keep the families in your prayer.”
Waukesha Police Chief Dan Thompson told reporters at a press conference that a “person of interest” was in custody and that a member of law enforcement did shoot at the SUV driver.
The sound of a weapon firing can be heard in footage from the parade, which shows the chaos that unfolded when the SUV broke through a roadblock.
An investigation is underway.
Additional reporting by The Associated Press.