Two buses taking children home from school have collided on a Tennessee highway, killing two pupils and an assistant and injuring another 27 people.
The children killed were about eight or younger, police said.
Knox County schools superintendent Dr Jim McIntyre fought back tears at a news conference, saying: "This is an unspeakable tragedy. This is what we work every day to try to prevent."
Police said a preliminary investigation indicated one bus made a sharp left turn, crossed over a concrete central reservation and hit the second bus, which was travelling in the opposite direction. The second bus flipped on to its side and slid.
Three seriously injured people were among seven taken to the University of Tennessee Medical Centre.
Another 20 children were being treated at East Tennessee Children's Hospital.
Knoxville police chief David Rausch said 18 of the 20 had been on a bus going home from Sunnyview Primary School, whose pupils are aged between five and seven. He described their injuries as "bumps and scrapes".
The other bus involved in the crash was from Chilhowee Intermediate School, whose pupils are aged between eight and 10. Mr Rausch said some youngsters from that bus were taken by their families to the children's hospital.
Classes were cancelled at the two schools today, but counsellors were being made available part of the day for pupils or families if needed, Dr McIntyre said.
Tennessee governor Bill Haslam, a former Knoxville mayor, broke off from a budget meeting to speak to reporters after hearing news of the tragedy.
"Any crash is a horrible thing when you have fatalities, and children involved," he said. "It's hard to imagine a worse situation. Our hearts go out to all the families impacted."
Forrest Robinson, who works at a petrol station and restaurant near the crash scene, said he heard a "really big bam". When he went to look, one of the buses was flipped and the other was "completely across the median".
He said parents soon arrived and some of them formed a prayer circle as they waited for news about their children.
The Knoxville News Sentinel said Mr Rausch notified relatives of one of the dead children at the scene, to screams and tears.