OAKLAND, Iowa — Questions about what caused the fatal school bus fire here last week remain unanswered at this point, but authorities have confirmed the cause of death for the driver and the student on board.

The Iowa Office of the State Medical Examiner determined that school bus driver Donald Hendricks, 74, and student Megan Klindt, 16, both died from “smoke and soot inhalation and thermal injuries.”

“The manner of death for the two decedents was classified as an accident,” John Kraemer, director of forensic operations, said in an emailed statement to School Bus Fleet.

Meanwhile, local and federal officials are continuing to investigate the Dec. 12 incident. Hendricks and Klindt were the only occupants of the Riverside Community School District bus. Authorities have said that the bus driver had apparently backed into a roadside ditch when the fire broke out, but it was not clear why he and his passenger were not able to evacuate from the bus.

The circumstances of the incident caught the interest of the National Transportation Safety Board (NTSB), which launched a full team investigation.

In a Dec. 14 press conference recorded by the Omaha World-Herald (see below), NTSB Senior Investigator Peter Kotowski said that the agency would try to determine the ignition source and the probable cause of the school bus fire. He noted that while NTSB 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.

Among other factors to be analyzed are toxicology samples from the driver and student, the school district’s transportation policies, the maintenance history of the bus, and the vehicle’s construction and emergency egress. NTSB was also working to determine whether the bus, a 2005 International Type C model, was involved in a safety recall for an electrical circuitry issue.

Kotowski said that about two weeks after the on-scene investigation, NTSB would release a preliminary factual report. The full investigation will likely take 12 to 14 months, and then the agency will issue a final report.

An NTSB spokesperson told SBF on Thursday morning that he did not have any additional updates about the investigation.

Watch the Omaha World-Herald’s video of the Dec. 14 NTSB press conference here:

Fire expert Fredrik Rosen, marketing manager at RISE Research Institutes of Sweden, told SBF that while he was not familiar with the details of the Iowa incident, bus fires often relate to component failures. Accordingly, RISE developed a voluntary certification system, SPCR 191, to support vehicle operator maintenance work.

“The basis of SPCR 191 certification is training,” Rosen said. “It is crucial that the maintenance staff, safety engineers, quality inspectors, and fleet managers achieve training focused on fire prevention and conducting fire risk assessments.”