The blaze broke out at approximately 2:30 a.m., sending smoke and flames shooting into the air in the 5200 block of West Centinela Avenue.
The fire engulfed the second floor of the vacant building. The intense heat led firefighters to enter into defensive mode.
Firefighters brought in three ladder trucks and focused on the roof. About 140 firefighters knocked down the blaze in just under three hours. No injuries were reported.
Officials said the building's small offices made it hard to put out the fire.
"It's very difficult to have these confining, small area closets and stubborn fires in the area. Being an exterior attack, it's hard to get into those interior, confined spaces," said Chief Deputy Daren Palacios with the Los Angeles Fire Department.
Fire officials were looking into a report of some homeless individuals on the roof of the building.
"When the fire department first showed up, they exited someone off the building. Then, the fire started to shoot up. So it was just massive, massive flames," said Tressia Mitchell, who witnessed the fire from a nearby bus stop.
An arson unit was at the scene of the blaze as the cause remains under investigation.
Due to the cleanup efforts following the fire, parts of La Cienega Boulevard and Centinela Street were closed to traffic. Authorities advised drivers to avoid the area and use alternate routes.