OAKLAND, Iowa — The cause of the recent school bus fire here — and why the driver and passenger were not able to escape — are still unclear, but an investigation update provides some new details on the incident.

School bus driver Donald Hendricks, 74, and student Megan Klindt, 16, died in the Dec. 12 school bus fire in a rural area east of the small town of Oakland.

On Friday, the National Transportation Safety Board (NTSB) released a two-page preliminary report on its ongoing investigation of the incident.

Hendricks was driving a 2004 International Type C (conventional) school bus owned and operated by Riverside Community School District. Around 6:50 a.m., he turned right from a gravel road onto a driveway to pick up Klindt, his first passenger of the day, at the farm where she lived.

According to NTSB’s report, as the bus backed out of the driveway, its right dual rear wheels crossed an earthen strip next to the road and dropped into a 3-foot-deep ditch. The wheels lost traction, and the bus became stuck, with about half of it sticking out across the gravel road.

NTSB said that as Hendricks tried to move the bus forward out of the ditch, a fire broke out in the engine compartment and spread into the passenger compartment. At some point, grass and vegetation near the back of the bus also caught fire.

What went on inside the bus remains a mystery.

“For unknown reasons, the driver and student passenger did not exit the school bus,” NTSB said in the report.

As previously reported, the Iowa Office of the State Medical Examiner determined that Hendricks and Klindt both died from “smoke and soot inhalation and thermal injuries.”

NTSB’s preliminary report notes that there were three recalls that applied to the school bus, to correct issues with a sensor for the antilock brake system, a sensor for the stop arm, and a wiring harness for the stepwell heater blower motor. Investigators confirmed that the three recalls had been performed.

The bus was equipped with a video surveillance system, but NTSB found that it was too severely damaged by the blaze to recover data.

The agency will continue to analyze various factors as it works to identify the probable cause.

“Additional examinations of vehicle components and post-fire evidence will be performed to determine the potential origin of the fire and its propagation,” NTSB said in the report.

In a press conference two days after the fire, NTSB Senior Investigator Peter Kotowski said that while the agency has investigated crashes that involved fires in school buses and motorcoaches, the Iowa incident was unusual because of the lack of a crash.

“The circumstances in this one, without a sustained or heavy impact, is one of the reasons why it drew our attention,” Kotowski said.

NTSB’s full investigation will likely take at least a year, and then the agency will issue a final report. The preliminary report is available here.