The fire in a three-story office building on the 1600 block of Webster Street was reported around 9:19 a.m., according to Battalion Chief Emon Usher.
Usher said the building was full of furniture and had a complicated layout with multiple levels, different types of construction and an auditorium at the back. While initial reports suggested the building was abandoned, Usher could not confirm that.
The fire was brought under control by 9:47 a.m., but one firefighter suffered minor burns to his face and neck, Usher said. He was treated at a burn center in San Francisco.
The exact origins and cause of the fire remain under investigation, but it is considered suspicious, Usher said.