Two school buses collided on a Tennessee highway on Tuesday, killing two children and an adult and injuring 23.

A police dispatcher told FoxNews.com happened shortly before 3 p.m., though she declined to provide further details. Knoxville Police Chief David Rausch told The Associated Press that the adult who died was an aide.

Three seriously injured people were taken to the University of Tennessee Medical Center and were in stable condition, Rausch said. East Tennessee Children's Hospital spokeswoman Erica Estep said another 20 children were being treated there for injuries that were not life-threatening.

Witnesses ran to the accident scene to help children off the buses, one of which was on its side, according to first responders. The children on the buses were reportedly from an intermediate school and a primary school.

“I heard the big bang,” Karla Corona, a parts sales manager, told The Knoxville News Sentinel. “As soon as I rang up the customer, we saw that a bus had flipped over and another was in the middle of the road."

Rausch said 18 of the 20 had been on a bus going home from Sunnyview Primary School, which serves kindergarten through second grade. He described their injuries as "bumps and scrapes." They were taken to the hospital on a city bus "to be checked out," he said.

The other bus involved in the crash was from Chilhowee Intermediate School, which serves third through fifth grade. Rausch said some children from that bus were taken by their families to the children's hospital.

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."

Some of the uninjured students were reunited with their parents at the crash scene.

There was no immediate word on what caused the crash or the exact condition of those who were injured. Shawn O'Neill, a forecaster for the National Weather Service, told FoxNews.com that light rain had been falling throughout the day in the Knoxville area, and there was light fog and mist as well.

FoxNews.com's Karl de Vries and The Associated Press contributed to this report.