Sgt. Rich Vierra says the pot plants, valued at about $500,000, were not burned during Monday's fire in the former Kar Mee Fortune Cookie Factory in Oakland's Chinatown district.
Police say the plants ranged in size from seedlings to 4 feet tall and were being grown in about 1,750 square feet of space.
The growers were apparently using an old power line outside the building to run the lights, ventilation and irrigation systems.
Authorities say the small blaze was sparked by an electrical malfunction.
The cookie factory's previous owners were evicted last year.