The source of a fatal ambulance fire in Kailua last year was an oxygen tank, an investigation found, but its cause remains unclear.

The Honolulu Emergency Medical Services today released a letter it received containing the findings of an investigation following the Aug. 24 ambulance fire that killed 91-year-old patient Fred Kaneshiro and critically injured 36-year-old paramedic Jeff Wilkinson, who were both inside the vehicle.

The letter was sent to EMS on Nov. 16., and the investigation, conducted by the Pennsylvania-based nonprofit Emergency Care Research Institute, or ECRI, confirmed initial findings that a portable oxygen tank regulator was the source of the fire.

The investigation could not determine the cause of the fire, however.

“At the time of this report, the precise mechanism that caused the fire within the regulator is not known,” Chris Schabowsky, director of ECRI’s Accident and Forensic Investigation Services, said in the letter. “Additional testing may provide more evidence to definitively determine the cause of the fire, but there is also a chance that it will not deliver enough information to offer more clarity.”

Schabowsky did say that certain contaminants in the oxygen tank and regulator assembly are known to cause fires because they burn when exposed to high pressure and heat in an oxygen-rich environment.

The regulator displayed significant physical damage, and Schabowsky said that the connection of the hose to the oxygen tank initiated the fire.

The investigation also detailed the events surrounding the ambulance prior to and during the fire, also confirming initial findings.

Wilkinson had been using wall outlet and a mask and hose to provide oxygen to Kaneshiro while the ambulance was on its way to Adventist Health Castle in Kailua.

While approaching the center’s emergency department bay, Wilkinson removed the hose from the wall outlet to a portable oxygen tank. Another paramedic, who had been driving the ambulance at the time, heard a “pop” noise the moment the hose was connected to the oxygen tank.

“This was immediately followed by a flash of bright light and fire,” Schabowsky said.

The ambulance then filled with fire and ambulance, “and the paramedic heard a continuous sound similar to the ‘noise of an activated propane blowtorch,’” Schabowsky added.

“We want to thank the public for its support of our EMS personnel during this difficult time,” EMS said in a statement. “We also continue to send our condolences to the family of the deceased patient. Paramedic Jeff Wilkinson is recovering at home and we keep him in our thoughts.”