Indoor pot farms in Northern California can be hard to spot until they catch fire and release fumes from chemicals and mold. The operations are often run by gangs and can involve violence, including a recent shooting death in San Jose. 

Illegal residential marijuana farms are hardly a new phenomenon in the Northern California Bay Area, but the numbers appear to be increasing at a time when law enforcement resources are dwindling and police are focused on more pressing priorities -- tracking violent crime, gang activity, and keeping a patrol presence on the streets, reports the San Jose Mercury News .

"I'm comfortable saying that there are over 100 of these houses, and the majority are in the city of San Jose," said Sgt. John Spagnola, who heads the two-man Marijuana Eradication Team for the Santa Clara County (Calif.) Sheriff's Office, the only South Bay agency with deputies devoted solely to eradicating marijuana. "We have tips right now that we just haven't had time to deal with yet. We could constantly be busting them."

What is clear is that when these houses go up in smoke, as they often do, the result is a nightmare for firefighters responding to blazes caused by marijuana growers' shoddy handiwork in bypassing electrical meters to stay off the radar of the electric company. Often, the houses also have chemicals and mold inside.

Indoor pot operations often are in otherwise safe, quiet residential neighborhoods. From the outside, they look like regular houses, hiding in plain sight until they catch fire, which happened twice within four days in San Jose in March.

In the South Bay, grow houses are often run by gangs affiliated with Vietnamese coffee houses, Spagnola said.