Five people have died and more than 40 are injured after a vehicle drove through a Christmas parade in the US state of Wisconsin.
A red SUV has hit dozens of people in the city of Waukesha, which is about 20 miles west of central Milwaukee, just before 5pm on Sunday.
One video showed a woman screaming, "Oh my God!" repeatedly as a group of young dancers was struck, while a father described going from "one crumpled body to the other" in search of his daughter.
Officials said a "person of interest" who may have a significant criminal history was being questioned overnight, adding that investigators were looking into the possibility the driver had been involved in an earlier incident involving a knife and was fleeing when the vehicle reached the parade, NBC reported.
The speeding SUV appeared to approach the parade's marchers from behind, and police chief Dan Thompson said a "suspect vehicle" had been found and that a "person of interest" was in police custody.
He said an officer had shot at the SUV in an effort to stop it as it crashed through street barriers.
Eleven adults and 12 children were taken to hospital after being hit by the vehicle and Mr Thompson confirmed there were "some fatalities". Many took themselves to hospital to be treated for injuries.
No one was injured by the police gunshots.
He said that, despite claims on social media, it did not appear that any shots were fired from inside the vehicle.
Corey Montiho, a Waukesha school district board member, told the Milwaukee Journal Sentinel that his daughter's dance team was hit by the vehicle.
"There were pompoms and shoes and spilled hot chocolate everywhere," he said.
"I had to go from one crumpled body to the other to find my daughter," he said.
"My wife and two daughters were almost hit. Please pray for everybody. Please pray."
Angelito Tenorio, who was also at the parade, told the newspaper: "As we were walking back in between the buildings... we saw an SUV cross over, just put the pedal to the metal and just zooming full speed along the parade route.
"And then we heard a loud bang, and just deafening cries and screams from people who are struck by the vehicle."
Chris Germain, co-owner of the Aspire Dance Centre studio, had about 70 people in the parade ranging from as young as two, up to 18, being pulled in wagons.
Mr Germain, whose three-year-old daughter was in the parade, said he was driving at the head of their entry when he saw a maroon SUV that "just blazed right past us".
He jumped out of his own car and gathered the girls who were with him to safety, before walking ahead to see what had happened.
"There were small children laying all over the road, there were police officers and EMTs (emergency medical technician) doing CPR on multiple members of the parade," he said.
A woman told Milwaukee's Fox6 TV station that the SUV hit a dance team of girls aged between nine and 15.
She said there was initially silence, followed by screaming, running, and efforts to check on those injured.
The Waukesha school district cancelled classes on Monday and said in a notice on its website that extra counsellors would be on hand for students and staff.
Local politicians expressed "horror" at what had happened, with Attorney General Josh Kaul, the state's top law enforcement officer, tweeting: "What took place in Waukesha today is sickening, and I have every confidence that those responsible will be brought to justice."
Waukesha mayor Shawn Reilly said: "Today, our community faced horror and tragedy in what should have been a community celebration.
"I'm deeply saddened to know that so many in our community went to a parade, but ended up dealing with injuries and heartache."
The Milwaukee Dancing Grannies posted on its Facebook page that "members of the group and volunteers were impacted and we are waiting for word on their conditions".
A Catholic priest, multiple parishioners and Waukesha Catholic schoolchildren were among those injured, the Archdiocese of Milwaukee spokeswoman Sandra Peterson said.
The parade, now in its 59th year, is an annual event held on the Sunday before Thanksgiving.
President Joe Biden was briefed by aides about the incident.
A White House official said: "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. We have reached out to state and local officials to offer any support and assistance as needed."
More on this breaking story as we get it.