OAKLAND, Iowa (AP) — A preliminary report on a school bus fire in western Iowa that killed the driver and a teenage passenger says the fire started in the engine compartment after the bus became stuck in a ditch.

The preliminary report from the National Transportation Safety Board was issued Friday.

The Dec. 12 fire killed 16-year-old Megan Klindt and 74-year-old Donnie Hendricks.

Friday’s report found that Hendricks was backing the bus out of Klindt’s rural driveway when the bus’ right rear wheels dropped into a 3-foot-deep ditch. As Hendricks tried to drive out of the ditch, a fire began in the engine compartment and spread to the passenger compartment.

The report says it is unknown why Hendricks and Klindt did not exit the bus.