Two school buses collided on a Tennessee highway Tuesday afternoon, killing three and injuring 23.

The crash occurred near Knoxville, at approximately 3 p.m., and two children and one adult died. Knoxville Police Chief David Rausch said that the adult who died was a teacher’s aide.

Rausch added that three passengers who were critically injured were transported to University of Tennessee Medical Center, and are in stable condition. An additional 20 students were taken to East Tennessee Children’s Hospital to be treated for non life-threatening injuries, according to hospital spokesperson Erica Estep.

First responders on the scene witnessed that one of the buses was on its side. Karla Corona, a parts sales manager, commented, “I heard the big bang. 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.”

2 kids and an adult killed in Tennessee school bus collision http://t.co/oX7NhvXIC1 pic.twitter.com/aj0uHtbz0q

— NBC News (@NBCNews) December 3, 2014

.@knoxschoolsupt thanks KPD, first responders, and the many people who assisted on the scene of the accident.

— Knox County Schools (@knoxschools) December 2, 2014

Rausch commented that 18 of the 20 of the students had been on a bus leaving from Sunnyview Primary School, which educates kindergarten through second grade. Rausch added that the students were taken to the hospital on a city bus to be checked out for “bumps and scrapes.”

The other bus was from Chilhowee Intermediate School, which serves third through fifth grade. Rausch said some of the passengers were taken to the hospital by their families.

3 dead in double school bus crash in Tennessee – http://t.co/ys1mi9HRlN pic.twitter.com/puPDoMkX0Z

— 10TV.com (@10TV) December 2, 2014

Knox County Schools Superintendent Dr. Jim McIntyre said at a news conference, “This is an unspeakable tragedy. This is what we work every day to try to prevent.”

No school tomorrow (Dec. 3) for Chilhowee Int. and Sunnyview Primary. Schools will be open 10a-2p for counseling support.

— Knox County Schools (@knoxschools) December 3, 2014

Special message from .@knoxschoolsupt on today's tragic bus accident at http://t.co/1wt7Ln5Xx0.

— Knox County Schools (@knoxschools) December 3, 2014