The deaths were connected to a nearby robbery, said Lt. Patrick Nelson of the Los Angeles County Sheriff's Department's homicide bureau.
A robbery occurred at a nearby Wal-Mart shortly after the house began burning, and the two incidents were connected, said Nelson.
He said investigators found "visible trauma" to the victims and that their bodies were found at separate locations inside the home, but he would not elaborate.
Nelson said the bodies appear to be of one adult and three young adults or teenagers.
It was unclear if the fire was set before or after the murders, he said.
Firefighters discovered the bodies Tuesday morning when they responded to a fire in the Lancaster area, according to the Los Angeles County Fire Department.
