Placentia police were investigating an illegal marijuana growing operation in an industrial building discovered when it caused a utility pole to catch fire, authorities said.

The discovery was made about 3:15 p.m. Sunday in the 700 block of South Melrose Street but officers were still dealing with the marijuana plants at 5 a.m. Monday and faced hours more work, Placentia police Sgt. Bryce Angel told City News Service.

"It's huge," Angel said. "They put up drywall and made a bunch of rooms for different sized plants."

More than 3,600 plants were found in what is believed to be a nursery.

The operation was described as a "pretty professional operation." Many of the interior rooms of the building had security systems, delaying police access.

Police do not know who is responsible for the operation, he said. They will try to track them down.

Placentia does not issue permits for marijuana cultivation or distribution, Angel said.

NBC4's Toni Guinyard contributed to this report.