ARI SHAPIRO, HOST:

While California recovers from the deadliest wildfires in the state's history, investigators are asking, how did this happen? Forensic teams are sifting through ashes and interviewing witnesses trying to find out who or what started the fires. Ron Eldridge of CAL FIRE has investigated fires in California for more than a decade and joins us now. Welcome to the program.

RON ELDRIDGE: Thank you.

SHAPIRO: In your experience, does the breakthrough often come from an eyewitness, from actual forensic evidence on the ground? How do you find out how something like this began?

ELDRIDGE: It's both. The investigators will start out looking at the indicators on the ground and what that leads them to, developing a theory as to what caused the fire and then reaching out to anybody and everybody that possibly could have seen, whether it was a vehicle in the area, whether it was kids in the area, whatever the case may be, you know, reaching out to those witnesses and gathering more and more information.

SHAPIRO: So take us into the room where people are discussing this day by day. How closely does it resemble, for example, crime scene investigations that we might have seen on TV or in the movies?

ELDRIDGE: It's very similar. Our whole job is based on scientific method and gathering information, and sometimes indicators are false, and there's lots of influences on those indicators, you know, from wind to firefighting efforts. Sometimes, you know, as the crews come in and start fighting fire, that's their priority, and sometimes that ends up disturbing what we're looking at.

SHAPIRO: California's Public Utilities Commission has asked the power company PG&E and six telecom companies to preserve all evidence connected to the fires. While we don't know whether power lines were responsible for starting these fires, how common would that sort of situation be?

ELDRIDGE: I wouldn't call it common. There's not a discernible pattern there. Each case is individual, and we look at where the evidence leads us to what the cause of the fire is.

SHAPIRO: Are these sorts of things more often caused by arson, downed power lines, a campfire that somebody didn't put out? What tends to be the most frequent cause of something like this?

ELDRIDGE: Most all of our fires are human-caused type fires.

SHAPIRO: That seems to encompass a lot of different things.

ELDRIDGE: Yeah. I mean, it's - that's everything, anything from an abandoned campfire to - there's all kinds of different human-caused fires. If not for the human element, the fire would not have started.

SHAPIRO: How often are you unable to conclude what caused the fires?

ELDRIDGE: Very seldom, but what does happen occasionally is maybe we can only narrow it down to two or three things. We can't say that it was this one thing. And so in those cases, sometimes fires we classify them as undetermined, but it's not that we didn't figure it out. It's that we can't rule out every single possible ignition source.

SHAPIRO: What do you think the biggest challenge is going to be in determining the cause of these fires?

ELDRIDGE: It's just the size. There's going to be a lot and a lot of people to interview that may have seen things prior to the fires starting or as the fires started. So it's just - it's going to take a long time to sit down with all those individuals and find out what they saw.

SHAPIRO: Does a long time mean months, years?

ELDRIDGE: It takes what it takes. It's kind of similar to, you know, reading a book. You don't judge the end of a book by the first three or four chapters. You have to go through and gather all that information and look at all the information and make sure that, you know, what you're seeing is correct and ultimately building that book. And then writing a final chapter is what the origin and cause of that fire is.

SHAPIRO: Ron Eldridge is deputy chief of law enforcement for CAL FIRE. Thank you for talking with us today.

ELDRIDGE: You're welcome. Thank you. Transcript provided by NPR, Copyright NPR.