Four students in Durham, North Carolina, suffered burns while aboard their school bus on Monday afternoon when the bus’ radiator overheated and spilled hot liquid on them.

In a letter to parents, Charlotte Wilson, the principal of the school, the Institute for Development of Young Leaders, explained that when the radiator overheated, it spilled hot liquid into the rear passenger compartment where the children were seated.

The children were taken to the hospital for treatment for burns sustained mostly to the lower limbs, according to the letter, and although three of the children had injuries that required them to be transported to the hospital’s burn unit, none were considered to be life-threatening. All the other students aboard the bus as well as the driver were cleared by first responders and boarded another bus for transportation home, according to the letter.  

The letter also stated that this is the first incident of this type the school has experienced, and that it will continue investigating the incident.   

School officials told WTVD that the company that owns the bus, Frontline Quality Transportation, inspected the bus five days before the incident occurred. As a charter school, its buses are not subject to annual state inspections, according to the news outlet. The state offers courtesy inspections, but there's no record of any such inspection at the Institute for the Development of Young Leaders, WTVD reports.