The fire was reported shortly after 8 a.m. at a two-story duplex in the 2300 block of Banyan Way, Contra Costa County Fire Protection District spokeswoman Emily Hopkins said.
Firefighters were initially told that there were four people inside the home who might need to be rescued and called for a second-alarm response and an ambulance, Hopkins said.
When they got there, fire crews found two people suffering from serious smoke inhalation. Both were taken to the hospital. Two other people, who lived in the other half of the duplex, had escaped the blaze uninjured.
McIvor, however, was trapped in an upstairs bedroom and firefighters were not able to reach her in time.
"We did everything we could, but we couldn't get to her," Battalion Chief Alan Hartford said.
Crews had the fire knocked down on the first floor by 8:23 a.m. and on the second floor by about 8:40 a.m., Hartford said.
The fire appears to have started near a spare oxygen tank in a stairwell. When the tank failed, the oxygen was released, fueling the fire and causing it to quickly climb the stairs, Hartford said.
By the time fire crews arrived, the second floor was engulfed in flames.
Fire investigators still don't know what the ignition source was, but Hartford said investigators haven't found anything to suggest that the fire was suspicious.
Firefighters were able to protect nearby homes, but the blaze destroyed the half of the duplex where it originated. The other half was damaged, but could eventually be habitable, Hartford said.
The American Red Cross is helping the two people who were displaced. The other two remained in the hospital this afternoon, officials said.