Preliminary findings from an investigation into an ambulance fire that killed a patient and injured a paramedic last month show the blaze originated in an oxygen device that is routinely used, officials in Hawaii said Wednesday.

The Aug. 24 fire killed a 91-year-old patient and severely injured a 36-year-old paramedic when flames engulfed the back of the ambulance in the parking lot of a Kailua hospital.

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"Based on the preliminary findings of this investigation ... the fire is classified as accidental and originated at the portable oxygen regulator assembly," Honolulu Fire Chief Sheldon "Kalani" Hao said at a news conference. "The exact and definitive cause of this fire cannot be determined within the scope of the Honolulu Fire Department."

Dr. Jim Ireland, the emergency services director for the city and county, said the injured paramedic reported hearing a loud sound when he was connecting a breathing device called a CPAP, which stands for continuous positive airway pressure, to an oxygen source in the back of the ambulance.

"It is reported that at the time the paramedic connected the CPAP oxygen line to the portable oxygen cylinder, there was a sound described as a pop, followed by a bright flash of light with the back of the ambulance immediately filling with smoke and fire," Ireland said.

He said the emergency medical technician who was driving the ambulance reported hearing the same sound before the fire.

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The city hired investigators from the Emergency Care Research Institute, a private, nonprofit firm that specializes in medical device evaluations, to help the fire department determine the cause of the fire.

Ireland said the investigation into what sparked the fire is ongoing and a final report will be issued once complete.