Crews were called to the 700-block of Terminal Avenue just after 6 a.m. PT to find the two-storey building in flames.
About 40 firefighters were l on the scene most of the day, Capt. Gabe Roder said.
There were no injuries and investigators were trying to determine if someone broke into the building or had been squatting there before the fire.
"It's a vacant building, so you would wonder why a fire would start," Battalion Chief Randy Hebenton told CBC News.