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How Do Fire Investigators Actually Investigate a Fire?

Officials say the five-alarm Mission Bay blaze appears to be have been started by welding equipment. How do they know?

About 150 firefighters and 90 fire trucks helped extinguish yesterday's five-alarm fire in Mission Bay. Thankfully nobody was seriously injured, but in the case of any multi-million dollar building burning to the ground, the question remains: How did this happen?

Early reports claim no evidence of foul play and cite a case of welding as the most probable culprit. An official report is expected for release later this evening at the earliest.

Well, that was quick. Impressively quick. And it got us wondering: how exactly does one determine the cause of fires? Within such a chaotic scene of debris and destruction, who's the guy that sorts through the ashes to finger the faulty wiring, the subtle gas leak, the well-placed match?

So we asked a real-life fire investigator how it's done. Brendan O’Leary of CCSF’s Fire Science Department served as a member of San Francisco’s Fire Department for 31 and a half years and in the Bureau of Fire Investigation for 15 years. We recently spoke with him to find out what happens in an investigation like this one.

The starting point in figuring out how a fire started is to figure out where the fire started. O’Leary said that they “start off broad and get more specific.” First investigators speak with witnesses and first-responding firefighters. Then they investigate the exterior of the building, noting fire patterns around windows and doors. The interior is similarly scanned for patterns. Armed with training in “fire dynamics,” they narrow down the fire’s origin to a floor, then to room, and to specific area. The key is tracking the progression of the fire. “Once we get a point of origin, we ask around to see who was last seen at that location and what was going on there.”

Unfortunately, in a blaze as big as the Mission Bay fire, much of this evidence is either completely charred or collapsed. In this case, O’Leary said that video footage is especially useful. Surveillance feeds of nearby buildings, news helicopters, even iPhone cameras are a big help. “With social media as big as it is these days, that can be helpful.” (Keep Instagramming those fires folks.)

The former fire investigator went on to say that construction zones are particularly dangerous. Filled with miscellaneous construction material and lacking in any wall insulation or sprinklers, they’re essentially giant tinder boxes. Throw in some welders, exposed electrical wiring, and gas-powered tools and you have many chances for a spark to turn serious.

O’Leary recalls that two of the biggest fires of his career also occurred at large construction sites: an unfinished apartment at 7th and Folsom in the 80's and another multi-story site on Divisadero in the early 90's. Both five-alarm fires. Both ignited from the hustle and bustle of a large construction site.

So what does that mean for fire safety in our new "build baby build" town? San Francisco has roughly 150 major real estate developments under construction (and another 145 or so going through the approval process). That’s a lot of tinder boxes sitting under the Bay’s natural bellows. Let's hope these developers are up to snuff on their fire safety codes.


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