The discovery was made around 7 p.m. after a fire broke out on the 1000 block of south Melrose Street near Melrose Elementary. Police say transformers on the side of the building blew up.
"Firefighters saw smoke coming from the roof of the unit that is in question. They made entry through the roll-up doors and found the plants," said Placentia Police Det. Corinne Loomis.
Police say the grow was intricate, with a hydroponics irrigation system. Nearly all of the 5,000 square feet of floor space in the building was covered with between 1,500 and 2,000 marijuana plants. Some of the plants were just a few feet tall; others were the size of a Christmas tree.
The street value of the grow is estimated at $500,000.
Some people who work nearby say they had their suspicions about the building.
"I used to walk on my lunch breaks this way, and it always smelled like marijuana over there," said Noel Benny, who works nearby.
Police will be using utility records to track down the building's owner. They say it's clear this operation had been going on here for quite some time.
"It's not going to attract a lot of attention," said Loomis. "It's not like a meth lab where you have fumes or dangerous things being done...but certainly for a city or community of our size, this is a substantial grow."
Anyone with information that can help police should call the Placentia Police Department at (714) 993-8164.