Two children and one adult died when two school buses crashed this afternoon in East Knoxville, Tennessee, authorities said.
The names and ages of the victims have not been released but law enforcement officials confirmed that the children were between kindergarten and third grade. The adult was an aide on the bus.
"This is an unspeakable tragedy," Knox County Schools Superintendent Jim McIntrye said at a news conference, calling the incident "just a terrible accident."
At least 20 other children were injured, Knox County school officials said.
The injured students were taken to East Tennessee Children's Hospital and Dr. Ryan Redman said that all of the injuries were non-life threatening.
They had all been released as of this evening, the hospital announced.
School officials confirmed to ABC News that one of the buses was carrying children from Chilhowee Elementary School and the other served Sunnyview Primary School.
According to a preliminary investigation, the bus from Chilhowee was traveling east on Asheville Highway when for some unknown reason it made a sharp left-hand turn and crossed over a concrete median, the Knoxville Police Department said in a statement this evening.
The bus then struck the side of the bus from Sunnyview, which was traveling west, according to police. The Sunnyview bus flipped onto its side and continued sliding until it came to a rest across both westbound lanes.
Two female children and one adult female from the Sunnyview bus were pronounced dead on the scene, police said. Two children and one adult were transported to UT Medical Center for treatment of multiple injuries.
Eighteen additional children from the Sunnyview bus were transported non-emergency to Children's Hospital, were suffering from cuts, scrapes, bruises and other non-life threatening injuries.
Several parents of students from the Chilhowee bus took their children to Children's Hospital for observation.
The driver of the Chilhowee bus was also transported to UT Medical Center for treatment of non-life threatening injuries.
Fatal Accident Investigators from the Knoxville Police Department were investigating the crash this evening, along with members of the Tennessee Highway Patrol, which will conduct an inspection on the buses, police said.
The NTSB was sending a crash investigation team to Knoxville to assist in the investigation.
Neither school will hold classes Wednesday. Instead, counselors will be available for students and families who need support.