The fire burned for nearly five hours on Saturday, destroying Healdsurg's main postal facility on Center Street.
Federal investigators were back at the scene Monday. They say it appears the fire was an accident, possibly from an electric short in the attic.
The building is a total loss, including all the mail that was inside. Officials say claims for damaged or destroyed mail will be dealt with on an individual basis.
The temporary facility is at the postal annex located on Foss Creek Circle.