The National Transportation Safety Board said investigators still do not know why a 74-year-old school bus driver and a 16-year-old student in Iowa were unable to evacuate the vehicle after a fire started in the engine compartment last month.
NTSB released its preliminary report on the Dec. 12 incident that occurred near Oakland, Iowa. Investigators determined that Donald Hicks, a veteran driver for Riverside Community School District, had just picked up that morning's first passenger, Megan Klindt, at her home.
Hendricks was backing out of the Klindt family driveway onto a gravel road when the rear-dual wheels of the 2004 IC Bus CE dropped into a three-foot ditch. Investigators said the rear drive wheels lost traction, despite not losing contact with the ground, and the bus became stuck.
As Hendricks attempted to move the bus out of the ditch, a fire started in the engine compartment and spread into the passenger compartment.
The fire severely damaged an REI onboard video recording system and data was unsalvageable, the NTSB report added.
Investigators continue to perform tests on bus components to determine the fire's origin and why it spread so quickly, as well as why Hendricks and Klindt were unable to evacuate. NTSB said it also is reviewing vehicle maintenance and state inspection records as well as driver health and training records.
"All aspects of the crash remain under investigation to determine the probable cause," the report concluded.
The bus was subject to three National Highway Traffic Safety Administration recalls. Investigators determined that issues with a sensor used in the antilock braking system, a sensor used in the side-mounted stop arm and a wiring harness to the stepwell blower motor were all fixed.
A spokesman for IC Bus confirmed for STN three days after the fatal fire that all recalls had been completed. Navistar, the parent company of IC Bus, is cooperating with the investigation.
The final NTSB report won't be released for 12 to 18 months. An NTSB spokesman said the Pottawattamie County Sheriff’s Office continues to assist in the investigation, as does the Iowa Department of Education.