Damian Dovarganes / AP

Just west of Whiskeytown, California, at the foot of Merry Mountain and less than a hundred paces from Clear Creek, Jim Engel and I stood on the side of Highway 299 looking out over a crescent-shaped basin with no name. The basin was circumscribed by two arcs: on one side, the highway; on the other, a narrow access road. A little more than a week earlier, it had been mostly green.

By the time we arrived, it was scorched almost to a monochrome. The landscape looked to me as though it may as well have all combusted in an instant. But to Engel, the California Department of Forestry and Fire Protection’s chief wildfire investigator in the northern part of the state, that basin and what remained of its contents held all the clues needed to pinpoint the specific origin and cause of the Carr Fire, which killed eight people and destroyed more than 1,000 homes around Redding in July and August.

Now, nearly four months later, Engel is overseeing the investigation into the origin and cause of the Camp Fire. Over the past week and a half, the fire has devastated Northern California: Over 12,000 homes have been destroyed; more than 150,000 acres have burned; at least 79 people have died; and, even now, nearly 700 are missing. Those tallies will likely continue to rise in the coming weeks, but it is already clear that the fire is the deadliest and most destructive in state history.

What remains unclear, though, is how exactly it started—and who should be held responsible. Even as the Camp Fire rages on, Engel and his team of investigators have begun sifting through the ash and evidence in search of answers to those questions.

Cal Fire’s 160-some full-time wildfire investigators are both firefighters and police officers. They carry guns, have the power of arrest, and, until presented with evidence to the contrary, treat every fire as a potential arson. As any investigator will tell you, though, most fires begin with folly, not malice. Sometimes, the cause is subtle: a cigarette dropped in dry grass or sunlight refracted through a shard of glass. Other times, not so subtle: An investigator in Southern California told me he once traced a blaze back to a stray round from a homemade cannon.

Wildfire investigators rarely find themselves in the spotlight; they are less visible and less celebrated than the firefighters and hotshot crews that battle the blazes on the frontlines. But in an era of increasingly destructive—and increasingly costly—wildfires, investigators’ work is more important than ever. The fires that tore across the state between October and December of 2017 cost nearly $12 billion in claims, the California Department of Insurance reported. And Citigroup analysts estimated on Wednesday that the Camp Fire alone could top $15 billion. It is in large part up to wildfire investigators to sort out who will foot the bill.

By policy, Cal Fire keeps wildfire investigations under tight wraps. Of course, that has not prevented speculation about the cause of the Camp Fire. PG&E, the utility company that state officials say was responsible for at least 17 of 21 major fires in Northern California last fall, has reported two power-line failures around the time of the fire, and some have begun trying to connect the dots. The company’s stock value has taken a roller-coaster ride since the fire broke out on November 8, plunging more than 60 percent in a week before pulling out of free fall and surging almost 40 percent on Friday. For now, though, Cal Fire has remained tight-lipped about the Camp Fire investigation. And it likely will for the foreseeable future; investigations into major wildfires often take the greater part of a year, if not longer.

When Engel and I drove out in his pickup truck this summer to see the origin point of the Carr Fire, though, it was just over a week old. Cal Fire wasn’t responsible for the origin and cause investigation because the fire began on National Park Service land, but Engel wanted to see the scene for himself anyway. He has been investigating the causes of wildfires for 20 years and quite literally helped write the book on wildfire investigation.

As Engel and I stood along the side of Highway 299 surveying the burnt-out basin below, white ash fell like a light snow though the smoky air. The leaves and grass were mostly gone; what moves in wind tends to burn in fire. Even in the breeze, the charred landscape was still.

Scanning the blackened crescent, Engel was thinking about three things: fuels, topography and weather. Those are the three basic ingredients of fire behavior, and the three basic factors that a wildfire investigator considers in reconstructing how a fire ignited and spread.

Usually, investigators begin by interviewing firefighters and civilians on the scene to define a general area of origin—a large perimeter, sometimes encompassing as much as two acres of land, that they’re pretty sure contains the ignition point. From there they work inward. When Engel and I arrived at the scene of the Carr Fire, the forest service investigators had already finished their on-site work. A pair of traffic cones and some yellow caution tape marked the origin area, and I could make out a dark scrape mark curving off the road. A metal guardrail beside the cones had twisted as its wooden legs burned out from under it.

In the early afternoon of July 23, a passing trailer with a flat tire had sagged so low it ground against the pavement and sparked. At least one of those sparks leapt into the roadside star thistle, which unseasonable heat and low humidity had primed for ignition. 21-mile-per-hour gusts offered extra encouragement.

Engel traced the fire’s spread away from the origin. A few yards to the left of the cones, he pointed out a stone wedged in the dirt. To its left, some unburned grass had survived; to its right, scorched earth. As the fire advanced on the stone, Engel explained, it diverged, like a stream flowing around a boulder. A week before our visit, other indicators, like ash and fallen grass stems, would also have pointed the way to the fire’s origin. These had since scattered with the wind.

Unlike humans, flames run faster uphill (heat rises), which is why investigators care about topography. But made to choose between favorable slopes and combustible fuels, fire will always pick the latter. Penned in by the unforgiving asphalt of Highway 299 on one side, the flames began to creep downhill.

Wildfires do not tarry. Usually, a flame front passes through a tree in under a minute, consuming what burns easily and sparing what does not. If the fire is growing as it advances, the backside of the tree will show greater damage than the front: In Engel’s lingo, the fire “comes in low and exits high.” When it moves downhill, the burn is more even because fires tend not to grow as they descend. But the Carr Fire did. On the slope down from Highway 299, the sides of the pine trees facing uphill had kept most of their needles, while the sides facing downhill were bare. From this, Engel inferred that as the flames edged down the slope, they reached higher and higher into the foliage.

But even as it grew, the Carr Fire did not careen straight downhill; it sidestepped. The evidence, Engel told me, was in the pine needles. The effect wasn’t so pronounced that I had noticed on my own, but when Engel pointed it out, it was evident right away—all of the pine needles pointed left. “Needle freeze,” he called it. As fire burns past a tree, the heat makes the foliage more pliable so that it bends more easily in the wind. Once the fire has passed and the foliage begins to cool, the leaves often hold their positions. So long as the flames move windward—and they usually do—the trees point which way the fire went, like traumatized witnesses.

When the Carr Fire reached the open field at the bottom of the hill, it started to intensify. The greater their intensity, the less flames leave in their wake. As the fire tore through the brush, it left nothing behind. “Complete combustion,” Engel said. The upward slope on the far side of the basin only added to its momentum.

Just one line of defense remained: the access road. Asphalt, of course, does not often burn. But fires have their ways of surmounting that difficulty. Wind can push flames across a road. It can also carry embers from one side to the other; sometimes, it can carry them many miles. Under the right conditions, the sheer heat of the fire can ignite fuels across a road without flames ever touching them. Engel could not say how the Carr Fire had overcome the access road. But one way or another, it had.

“It probably held on this road for a little while at least before it got over,” Engel said matter-of-factly. “Then it was off to the races.”

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