OMAHA, Neb. (AP) — Federal safety investigators say enforcing better school bus driver standards and installing a fire suppression system may have prevented the deaths in a 2017 school bus fire in western Iowa. National Transportation Safety Board Chairman Robert Sumwalt said Tuesday that the Riverside Community School District didn’t ensure the driver was medically fit before the crash.

Sumwalt says 74-year-old bus driver Donald Hendricks, who died in the fire along with his 16-year-old passenger Megan Klindt, had mobility issues that limited his ability to evacuate the bus. Hendricks used a walker and had back surgery scheduled the week of the fire. Investigators said the fire likely started in the bus’ turbocharger, which overheated after the bus’ rear tires became stuck in a drainage ditch.

Sumwalt says having an improved fire suppression system could have provided more time to evacuate. The federal requirements for school bus fire safety haven’t been updated since 1971.