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Two students and one teacher’s aide are dead after two school buses collided on a Knoxville, Tennessee road Tuesday afternoon.

At least 27 others were injured in the Asheville Highway crash, authorities say. The children killed were in third grade or below, according to Knoxville Police Chief David Rausch.

It is unclear what caused the collision, but a preliminary investigation indicated one bus made a sharp turn, crossing over a concrete median. The second bus flipped on its side and slid as a result of the impact.

Three seriously injured people were taken to the University of Tennessee Medical Center and were in stable condition, Rausch said. Medical Center spokesman Jim Ragonese said a total of seven people were treated there, and three of them had been discharged by Tuesday evening.

East Tennessee Children’s Hospital spokeswoman Erica Estep said another 20 children were being treated there for injuries that were not life-threatening.

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.

Holding back tears at a news conference Tuesday afternoon, Knox County Schools Superintendent Dr. Jim McIntyre addressed the accident, calling it an “unspeakable tragedy.”

“This is what we work every day to try to prevent,” he said.

Our thoughts and prayers go out to the families of the victims.