SAN FRANCISCO (Map, News) - Firefighters tackling a three-alarm blaze in the Mission district on Thursday night stumbled upon an unexpected discovery while fending off the flames — a room filled with marijuana plants.

San Francisco Fire Department spokeswoman Lt. Mindy Talmadge said firefighters on the scene at 1142 S. Van Ness Ave. came across a “horticultural type of thing” on the building’s top floor.

Police Department spokesman Sgt. Steve Mannina confirmed that 25 to 30 marijuana plants were confiscated from the house. No criminal charges have yet been filed, although police are investigating the case, Mannina said.

Although an investigation into the fire is ongoing, Talmadge said early reports indicate that the blaze was caused by an electrical malfunction from an overload of circuits. Talmadge said there was a possibility the overload came as a result of appliances used in the top floor that housed the marijuana plants.

Thursday’s blaze originated at 1142 S. Van Ness Ave., which incorporates the top two floors of the building, before spreading down to 1140 S. Van Ness Ave., which inhabits the bottom floor, according to Talmadge. It was reported at 7:45 p.m. Thursday, contained by 8 p.m. and under control by 8:45 p.m., Talmadge said.

The owner of the house, Nobuo Yamaji, said he rented out the 1142 S. Van Ness Ave. apartment to an artist, but would not provide any further information.

The inferno at the three-story Victorian-style home caused $750,000 in structural damage, and $115,000 in damage to contents inside the home, according to Talmadge, who said no other buildings were damaged by the fire. One person was displaced, Talmadge said.

A nearby neighbor of the house, who declined to be identified, said he had seen men carrying large plants out of the building in the past, although he did not know if they were marijuana plants.

wreisman@examiner.com