Federal investigators say a fatal school bus fire in western Iowa last month started in the engine compartment after the bus backed into a ditch and became stuck.

The bus driver, 74-year-old Donald Hendricks, and a 16-year-old student, Megan Klindt, both died in the fire in rural Pottawattamie County on December 12. A report issued Friday by the National Transportation Safety Board says it’s “unknown” why the two were unable to exit the bus.

The accident happened as Hendricks was backing out of Klindt’s driveway. She was his first passenger of the day. In addition to a fire in the engine compartment, the report shows “grass and vegetation” at the rear of the bus also caught fire. The Riverside Community School District bus was subject to three recalls and each of them were performed, according to the NTSB’s preliminary report.