It took 69 firefighters about 40 minutes to extinguish the blaze. Once it was put out fire crews were able to pinpoint the origin.
Arson investigators were on scene all morning, trying to figure out how the blaze was started.
"[There was] heavy fire damage inside the medical marijuana plant, clinic, so that's the point of origin and that's where they're focusing their investigation," said Capt. Steve Ruda of the L.A. City Fire Department.
Firefighters believe the blaze started in a back office in the dispensary.
"Its office that's broken up into three separate areas. It was burning in the one in the back, got into the attic area and was working its way this way," said Capt. Lance McCloskey of the L.A. City Fire Department.
A specially trained accelerant sniffing dog was used to investigate the scene. Firefighters had to use masks and ventilation fans to clear the air.
"The smoke is what kills people, now you add to it, you know, the possibility of a substance that could possibly injure or hurt or cause medical conditions to firefighters. We have to be concerned about that," said Capt. Ruda.
Neighboring business owners believe the fire could have started accidently.
The seven other businesses inside the building will have to be relocated temporarily until the building is brought back up to code.