The investigator's report said the exact cause could not be determined but contamination or particulates within the oxygen cylinder could have caused the fire.
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The fire killed a 91-year-old patient being transported to a hospital. Emergency services said it continues to send its condolences to the family of the patient.
The fire also critically injured a Honolulu paramedic, who is still healing.
Hawaii released investigation findings into an ambulance that caught on fire last year, which killed a 91-year-old patient.
"Paramedic Jeff Wilkinson is recovering at home and we keep him in our thoughts," EMS spokesperson Shayne Enright said in a statement.
The report said investigators quickly determined the fire originated from an oxygen source because of the widespread damage within the ambulance box. No other devices were being used when the fire began, the paramedic told investigators.
Investigators didn't find any other readily identifiable ignition sources like faulty electronic equipment.
The report said oxygen regulator fires are rare but they have received the attention of government agencies and other organizations.