(CNN) -- Five people were killed and more than 40 were injured when a vehicle drove into a Christmas parade Sunday afternoon in Waukesha, Wisconsin, city officials said.
"These numbers may change as we collect additional information. Many people have self-transported to area hospitals," the city of Waukesha said in a Twitter post early Monday.
Witnesses described the horror of seeing individuals struck and lying on the ground after a red SUV drove through a series of barricades and barreled into the crowded parade route on Main Street around 4:39 p.m. local time.
Police Chief Daniel P. Thompson said officers are working with the Waukesha County Medical Examiner's Office to identify victims.
Thompson says there is "a person of interest in custody," but would not say whether it was the driver of the SUV nor if any arrests have been made.
"This is still a very fluid investigation," the chief told reporters Sunday night.
Children are among the injured, he said.
A Waukesha police officer discharged their weapon in an attempt to stop the driver, and no bystanders were struck by the officer's gunfire, the chief said. Thompson added that he doesn't believe any shots were fired from the vehicle.
It is unknown at this time whether Sunday's incident was an act of terrorism, Thompson said. "The scene is now safe and secure," he added.
Aurora Medical Center-Summit, a hospital in Waukesha County, told CNN Sunday night it is treating 13 patients: three listed in critical condition, four in serious condition and six in fair condition. "We are deeply saddened by today's senseless tragedy and our hearts are with the Waukesha community," a statement from the hospital said.
Children's Wisconsin, a pediatric hospital in Milwaukee, said it was treating 15 patients who were hurt, with no reported fatalities as of Sunday night. Waukesha is located about 20 miles west of downtown Milwaukee.
Froedtert Hospital in Milwaukee -- the only Level I trauma center in southeast Wisconsin -- confirmed to CNN that it is also treating patients but is not providing a specific number nor conditions. Level I trauma centers typically care for the most critically ill patients.
Witnesses describe a joyous night that ended in tragedy
A video obtained by CNN shows the moment the vehicle struck pedestrians. A red SUV is seen hitting an individual participating in a marching band, then the vehicle continues forward, hitting and running over others in the band and crowd before driving away.
Angela O'Boyle, who lives in an apartment on the fifth floor which overlooks the parade route, told CNN she was on the balcony watching when "the next thing I heard were screams, and turned my head and saw the car come and plow into the band that was just pass my balcony at that point.
"It hit at least two people right away and rolled over them. And then continued down the road to People's Park which is at the end of the block -- and then kept going, it didn't stop."
Boyle said she heard screaming and people yelling out their children's names, adding "it was not something that I wanted to see ... a little freaked out."
Kaylee Staral, a Milwaukee Journal Sentinel intern who witnessed the incident, told CNN's Pamela Brown "probably around 20 to 30 minutes into the parade, a red SUV came running down the middle of the street -- there were a lot of screams -- and we almost thought maybe it was Santa, but it was a red SUV and it hit a lot of people."
Staral went on to say "there were multiple people on the ground."
In her description into those injured, Staral said "right in front of me in the little spot where I was sitting, probably four people were down, so there were a lot of people that ran to go check on them. I believe that the four people in front of me were still breathing but they were down, they were not moving."
Schools closed as a community mourns
Roads in the area will remain closed for at least 24 hours, Thompson said. Businesses along roadway where the incident took place should remain closed Monday, city officials said.
Waukesha police had sent a push alert to cell phones in the area after the incident, asking those "within a 1/2 mile radius of the 5 points downtown" to immediately shelter in place. The alert is no longer in effect, Thompson said.
The School District of Waukesha canceled classes on Monday, according to the school district, and will determine if classes will resume Tuesday.
"The District will have additional counselors available during the school day at all buildings for all students that may be in need of support services," school officials said in a statement.
Waukesha County executive Paul Farrow said he "had just finished participating in the parade with my wife when I noticed police sirens." Farrow asked for prayers for the community and said he has been in touch with Mayor Shawn Reilly and Gov. Tony Evers.
The governor said on Twitter, "Kathy and I are praying for Waukesha tonight and all the kids, families, and community members affected by this senseless act. I'm grateful for the first responders and folks who acted quickly to help, and we are in contact with local partners as we await more information."
Waukesha alderman Don Paul Browne told CNN he was marching in the front of the parade and was out before the incident, saying "this is so sad and so senseless."
The White House is in touch with state and local officials, according to a White House official, and President Joe Biden has been briefed.
"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," a White House official said. "We have reached out to state and local officials to offer any support and assistance as needed."
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