Investigators said they believe the victims died before the fires started, The Oregonian reported Sunday.
Firefighters responding to a blaze Saturday found the body of Kathleen Nelson, 59. Bodies of two more people were found as firefighters worked to put out a fire at the house next door.
Those remains have been tentatively identified by police as the home's owners, Renae and Lola Cottam, both 73.
The Cottams' house was destroyed, police said. The Nelson home did not receive extensive damage. Authorities said both fires were intentionally set.
Edward Lawrence Nelson, 65, the husband of Kathleen Nelson, was arrested later in Douglas County by Oregon State Police. He is accused of aggravated murder and first-degree arson.