Fire Capt. Fred Herrera’s job is to go back to the beginning, the moment when flames began sucking up oxygen. A home, for instance. Or a car or an office building or a body.

Herrera, a 30-year veteran of the San Diego Fire Department, is one of several investigators who spend their days sifting through the ashes of other people’s lives. They poke through layers of blackened remains in search of a telltale burn mark, a funny-looking drip or a cigarette butt with the arsonist’s DNA.

Two investigators are on duty at all times, and they often have plenty to do. Arsonists ignite about 350 fires in the city a year, and there may be more cases that investigators can’t prove as arson.

I asked the Santee native about the evolution of fire investigations, the most common misconceptions about how fire works and the reasons people set things aflame.

How have fire investigations changed over the more than two decades you’ve been doing it?

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When I came in in the ’80s, there were a lot of assumptions considered to be true. In the ’90s, the scientific community began testing the data, and it was found that a lot of those things weren’t true.

It was thought 20 years ago that if the concrete foundation of a house had portions that had chipped away — something called spalling — that indicated someone had poured gasoline on that spot (a sign of arson). The heat generated would cause the concrete to spall like that.

But testing showed that when you pour gasoline (on fire) on a concrete slab, that was actually an area that was cooler than surrounding areas. That’s because liquid can never get hotter than its boiling point. The reason that the spalling was happening was because the whole area was superheated, and when the fire hoses came in, the cold water would hit the concrete, and an instant contraction would cause stress and chip away.

What else have fire investigators learned about how fire works that they didn’t used to know?

The mindset was that if window glass was covered with soot and if you could very easily wipe it off, that meant it was a fast, flashy fire, possibly an indication that gasoline was involved. The converse was that a cigarette fire that would smolder and bake soot on the glass, almost like a layer of paint.

While those two things are not untrue, there’s also a lot of other things that can influence fire behavior and make both of those things happen regardless of what the cause is. We now use those patterns like we use burn patterns on walls, wood or furnishings: to show where the fire came from and where it went to.

How did forensic scientists come up with these new conclusions about how fire works?

The most influential forensics were literally just building rooms in a laboratory setting, like a large warehouse, and furnishing them like a normal bedroom would be furnished and lighting them on fire and watching what happened. A lot of things were learned merely by observation.

Do you ever worry about getting a case wrong in the past based on faulty assumptions?

Myself, not so much. I was in on the beginning of a curve of forensics becoming a large part of our industry.

A 1991 arson case out of Texas has gotten tremendous attention because of allegations that a faulty arson investigation sent an innocent man to be executed. The case raises questions about the validity of assumptions about arson. How has it affected your world?

Fire investigators weren’t awakened by that incident. It was something we’ve been aware of: For many years, we’ve been conscious of the need to prove our process and use a methodology that’s testable and can withstand a challenge. A lot of the older assumptions were shown to be wrong. By making fire investigation more scientific, by doing the testing and observing fire behavior and confirming these things through instrumentation and laboratory analysis, we’ve gained confidence that we’re making the correct calls.

Have defense lawyers been going after arson investigators more often?

It’s more routine that we’re being challenged. Twenty years ago, the legal community didn’t know what to ask. Nobody understood it.

What’s the main motivation for people who set fires?

The most common reason that people start fires is out of spite or anger. It’s a man and wife, or boyfriend and girlfriend, or one gang against another gang, or a customer against a business owner. It’s out of emotion, which is good for us because it makes it easier for us to solve. The parties involved generally have an idea that there’s an ongoing dispute, giving us a road to go down.

There are those who light fires just for the enjoyment, and since there’s no rhyme or reason for that, they’re much more difficult to solve.

What about people who set fires for a thrill? Do you have a sense about what’s behind their actions?

It’s well documented that some people use fire to feel cleansed. They have a compulsion that until they light a fire, they just can’t calm down. When they light a fire, they feel cleansed and calm. And then when it starts to build up, eventually they have to start another fire.

Some people just like the attention, the fire engines showing up and the excitement. But a lot of people never say what’s on their mind.

What do people misunderstand about fire?

People will see fire in one area of the house, and they’ll run out and then they have a second thought. They think they could go in and get their purse, their keys or their dog. They think they have time but they realize, sometimes too late, that there’s no safe time period to be inside.

People don’t realize how quickly a fire will get from one area to another, especially with the fuels that we have in modern furnishings. Twenty, 30, 40 years ago, everything was cotton, linen and canvas and all of these natural fibers that burn relatively slowly. Now everything is plastic, and it burns and liquefies, becoming a pool of ignitable liquid.

What’s another myth about fire?

If a fire starts in a corner of a room, the general thought by the lay public is that it will burn in the corner, then spread to something next to that, and slowly work its way across the room. That gives you the impression that if you’re on the other side of the room, you’ll have time before you have to get out.

But in reality, all of the heat is going up and hitting the ceiling, equalizing across the whole room and heating it. It’s like pouring liquid on the floor, but backwards.

As soon as everything heats up to the ignition temperature for ordinary combustibles, flashover happens and they all go off at more or less the same time. The progression is very fast. In the average bedroom, it’s common to have flashover three-to-four minutes after a fire starts. It’s like a big broiler.

Please contact Randy Dotinga directly at and follow him on Twitter:

Randy Dotinga

Randy Dotinga is a freelance contributor to Voice of San Diego. Please contact him directly at

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