OAKLAND, Iowa (AP) — An autopsy blames a fire for the deaths of a driver and teenager who were killed when a school bus burst into flames in western Iowa.

An autopsy from the Iowa State Medical Examiner released Wednesday found that 16-year-old Megan Klindt and 74-year-old Donnie Hendricks died Dec. 12 from smoke and soot inhalation and thermal injuries.

Their deaths were found to not be related to injuries from a crash.

It's unclear what caused the bus to burst into flames as it backed out of a driveway and into a ditch. The fire happened shortly after Klindt was picked up at her family's farm southeast of Oakland, about 35 miles (55 kilometers) east of Omaha, Nebraska.