Investigators looking at the possibility that the fire victim may have been smoking near an oxygen tank.
Neighbors reported hearing an explosion right before they saw flames beginning to shoot out of the home in the 500 block of Key West Drive in Williamstown.
The home is in the Holidae City retirement community.
A woman in her early 90s lived there with her daughter. Her daughter wasn't there when the fire broke out around 10:30 p.m. Wednesday.
The elderly woman died in the blaze. As the flames spread to two more houses, firefighters called a second and then a third alarm. They were able to bring the fire under control in just over an hour. By then, though, the fire had jumped from one attic to the next of the closely-spaced homes.
A firefighter received medical treatment at the scene for exhaustion.
