The fire was reported just before 4:00 am on Seventh Street near Hoag.
Firefighters say a door to the building was damaged with light smoke going into the office. The smoke was blown out by fire crews.
A person was detained at the scene, but it's unclear if they were connected to the fire.
In a message from the district, officials said services in the building are temporarily "inaccessible" while firefighters conduct an arson investigation.