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(CNN) Police reported "some fatalities" after more than 20 people, including children, were struck by a vehicle that ran through a crowd at a Wisconsin holiday parade, Police Chief Daniel P. Thompson said in a press conference.
Authorities are working with the Waukesha County Medical Examiner's Office to confirm the identities of the deceased, Thompson said. No further information regarding the fatalities will be released while police notify their loved ones.
"This is the most important thing, is the families of this tragic event and making sure that the next of kin is conducted," he continued. "That is our priority in this tragic incident."
Authorities reported 11 adults and 12 minors were transported to area hospitals by ambulances and officers after a red SUV drove through the Waukesha Christmas Parade, Thompson said.
A police officer who has been with the department for more than six years discharged his weapon at the suspect's vehicle to try and stop it, according to Thompson.
"No bystanders were injured by a result of the weapon discharge," he said.
A suspect's vehicle has been recovered and a "person of interest" is in custody, the chief added. There is no active threat in the area and "the scene is now safe," Thompson said.
It is unknown at this time whether the incident was an act of terrorism, he added.
People were seen on the ground injured after the incident, Milwaukee Journal Sentinel intern Kaylee Staral, who was at the parade, told CNN's Pamela Brown Sunday night.
About "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 said.
The White House is monitoring the situation and President Joe Biden has been briefed, according to an administration official. The Federal Bureau of Investigation (FBI) Milwaukee field office is also aware of the incident, according to an FBI spokesperson.
Special Agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) have responded to assist the Waukesha Police, according to Erik Longnecker, Acting Division Chief of the Public Affairs Division of the ATF.
The School District of Waukesha has canceled classes on Monday, according to the school district, adding counselors will be available for "students and staff in need of support services."
Children's Wisconsin -- a pediatric hospital in Milwaukee -- says it is treating 15 patients who were hurt in the Waukesha holiday parade.
"Children's Wisconsin immediately prepared for a surge of patients after receiving notification from law enforcement of the incident at the Waukesha Holiday Christmas Parade," the hospital said in a written statement. "As of 8 p.m., Children's Wisconsin received 15 patients from the incident, with no reported fatalities at our hospital at that time."
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.
Froedtert Hospital in Milwaukee -- the only Level I trauma center in southeast Wisconsin -- confirmed to CNN that it also is treating patients from the Waukesha incident but is not providing a specific number nor conditions. Level I trauma centers typically care for the most critically ill patients.
Among the injured are a Catholic priest, multiple parishioners and Waukesha Catholic school children, according to Sandra Peterson, the communication director for the Archdiocese of Milwaukee.
"Please join us in prayer for all those involved, their families, and those who are traumatized from witnessing the horrible scene," Peterson said in a statement.
Video of the parade published on the city's Facebook page shows a red SUV driving quickly through the parade, followed by a police officer running through the scene. In the clip, the crowd is seen and heard reacting to events off-screen.
"The horrific violence at the Waukesha Christmas Parade is just heartbreaking," Wisconsin Sen. Tammy Baldwin wrote on Twitter. "Please keep the victims in your prayers tonight."
In her description of 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," she added.
Waukesha Police are advising people to avoid the downtown area and have established a family reunification center, the agency said on its official Facebook page
"This is an unspeakable tragedy, affecting us all as we work to overcome an extremely challenging two years and resume our cherished holiday traditions," Waukesha County Executive Paul Farrow said in a statement.
"I had just finished participating in the parade with my wife when I noticed police sirens. Shortly afterward, I learned that a vehicle drove through the parade route injuring many people. Please pray for our community tonight after the horrific events at the Waukesha Christmas Parade."
Farrow added officials are still investigating the incident to understand what happened and asked for prayers for the community.
"Kathy and I are praying for Waukesha tonight and all the kids, families, and community members affected by this senseless act," Wisconsin Governor Tony Evers said on Twitter. "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."