KNOXVILLE, Tenn. >> Two buses bringing children home from school collided on a Tennessee highway Tuesday afternoon, killing two students and an adult and injuring another 27 people.

Police said a preliminary investigation indicated one bus made a sharp left turn, crossed over a concrete median and hit the second bus, which was traveling in the opposite direction. The second bus flipped onto its side and slid.

The children killed were in third grade or below, said Knoxville Police Chief David Rausch. 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. 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.

Knox County Schools Superintendent Dr. Jim McIntyre fought back tears at a news conference. "This is an unspeakable tragedy," he said. "This is what we work every day to try to prevent."

Class was not being held at the two schools Wednesday, but counselors were being made available part of the day for students or families if needed, McIntyre said later in a statement.

Forrest Robinson, who works at a gas station and restaurant near the crash, 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 reported that Rausch notified relatives of one of the deceased children at the scene, to screams and tears.

Karla Corona, a manager at an AutoZone near the crash, told the newspaper that she heard a bang and then saw people running toward the buses.

"I saw all the kids being rushed off the bus. Firefighters were the first to show up, and two or three cop cars. People actually got out of their cars and ran to help," she said.

Tennessee Gov. Bill Haslam, a former Knoxville mayor, was holding a budget meeting on Tuesday. He spoke to reporters about the crash after hearing the news.

"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."

Associated Press Writer Travis Loller in Nashville, Tennessee, contributed to this report.