Fire Chief Richard Arruda says the couple had about 45 gallons of gasoline in nine plastic jugs stacked in a closet that also housed an air conditioning unit.
The gasoline fumes apparently set off Thursday's fire, which was quickly put out by a sprinkler system.
No one was seriously hurt, but residents of eight units of the complex were displaced.
Investigators believe the couple was storing the gas because of rising prices. Officials say they might face civil penalties for violating the fire code.
