CALISTOGA, Calif. >> On a blackened hillside across the road from Napa Valley’s prized vineyards, investigators have cordoned off driveways and posted round-the-clock security guards, forming a protective seal around what is believed to be the origin of the most destructive fire in California history.

“It’s a crime scene for us until we determine otherwise,” said Ron Eldridge, the deputy chief of law enforcement for Cal Fire, the agency leading the investigation. “Was there negligence or was there a violation of law? That’s ultimately what we are trying to determine.”

Among the thousands of wildfires recorded in California in recent years, most have been caused by human activity: sparks from rocks sliced by lawn mower blades; children playing with fire; arson; fireworks; welding torches; even satanic rituals.

For the state’s 160 full-time fire investigators, many of whom have been sent to the suspected points of origin of 17 fires in Northern California, the stakes have never been higher.

Determining the causes of the fires could have huge financial implications in deciding who ultimately pays for the extensive damage, including almost 8,000 structures destroyed. Insurance companies will be looking to recover some of the more than $1 billion that the California insurance commissioner estimates they could end up paying out.

Eldridge, the Cal Fire deputy chief, says his officers are not favoring one theory over another. They are interviewing witnesses and may call on experts, including metallurgists and arborists. Software that re-creates what an area looked like before the fire could also help in the search for the origin, he said.

“They have satellite imagery, they have aircraft, and they have boots on the ground,” said Andrea Buchanan, the chief deputy fire marshal for the city of Alexandria, Virginia, and a 37-year veteran of fire investigation. And, she added, they will probably need every tool they’ve got.

Experts who have studied the causes of wildfires in California say that it is too early to come to conclusions, but that the circumstances — nighttime ignition coupled with fierce winds — raise the possibility of power lines being involved.

“If there are high velocity winds, there’s every reason to suspect that power lines are a source,” said Jon E. Keeley, a fire expert with the U.S. Geological Survey in California. “We have many documented cases of power lines igniting fires during these high wind events.”

The utility company that runs power lines in the affected areas, Pacific Gas and Electric, said it was complying with a request from regulators to preserve all potential evidence.

The company trims or takes down 1.2 million trees near its power lines every year, a spokesman, Keith Stephens, said. California regulations require all branches within 4 feet of lines be removed.

Good fire investigators start on their bellies, looking at the tiniest things that burned — the blades of grass, said Jon Skinner, who oversees fire investigation and recovery for the federal Bureau of Land Management.

Grass-stem indicators, as they are called, can tell investigators a lot about the pace and direction of a fire and provide clues about where it began, Skinner said.

“If you have tall pieces of grass and a fire is just starting and burning right next to the ground, it will burn the bottom of that grass off first and grass stems will fall toward the flames,” he said. “But if the fire is ripping through the grass, it will burn the tops of the grass first.”

Truly violent wind events, though, can complicate the investigations, fire experts said. If multiple fires were being ignited around the same time — for example, with multiple power lines going down — it would be harder to determine whether one spread from another or whether they started independently.

In the case of this month’s fires, investigators are treating the 17 suspected ignition points separately. But many of the fires merged, and it remains possible that embers from one fire could have started what are currently being classified as separate fires.

In the Chimney Tops fire of November 2016 that spread out of Great Smoky Mountains National Park in Tennessee, investigators initially blamed two teenagers who had been playing with matches in the park. That fire, driven by 80 mph winds — comparable with what firefighters and evacuees faced in Northern California — ultimately roared into Gatlinburg, Tennessee, killing 14 people and destroying more than 2,500 structures.

But after a lengthy investigation, the local district attorney general, James B. Dunn, said this summer that the wind had so complicated the fire’s path and destruction that linking it back to one source was impossible. Criminal charges against the teenagers were dropped.

“Once the investigation confirmed multiple fires with multiple points of origin, it became impossible to prove which fire may have caused the death of an individual or damage to a particular structure,” Dunn said in a statement at the time.

Stephen J. Pyne, a fire historian at Arizona State University, said wind and electricity were an increasingly alarming combination in places where urban and wild lands bump up together, and it was one, he said, that had no cheap or easy fix.

Sparks from locomotives were once notorious for starting fires, he said, and “power lines are today what locomotives were a century ago” for exactly the same reason — they transit through things that can burn.

“The power of fire resides in the power to spread,” he said.

There is no doubt that power lines came down around the vineyards on the edges of Calistoga, California.

On Bennett Lane, where the deadliest of the fires is believed to have started, wires littered the roads even a week after the fires began. But whether the power lines provided the spark that began the fire remains an open question.

On the evening of Sunday, Oct. 8, the weather on Bennett Lane had turned violent. The winds pelted cars with pebbles and toppled a metal shed at a nearby winery.

The power went out in the home of Jim Pence, a retired schoolteacher who has lived on Bennett Lane for nearly four decades. An hour later, about 11 p.m., he decided to evacuate.

“I told my wife, ‘There’s no way a fireman can control that kind of wind.’” He unplugged his desktop computer, put it in his car and drove off.

Chrisdondie Palting, a Filipino vineyard worker who lives nearby, said the winds nearly knocked him over.

“You couldn’t stand up straight,” he said.

Directly across from the hill, Paul Block, an artist who converts used wine barrels into furniture and household items, woke to the sound of sirens. Across the street, the mountainside was on fire and the flames looked like they were in a wind tunnel.

“The flames were 20 feet long and they were horizontal,” Block said.

He saw the flames — which firefighters would name the Tubbs Fire, after a nearby street — jump the road and head up the mountainside toward Sonoma County, over the hills where it would ravage parts of Santa Rosa, the county seat.

“It started here,” Block said, referring to hill across the street. “This was the starting line.”

Jorge Gutierrez, the property manager at an estate just north of the hill where the fire is believed to have started, scrambled to save horses, chickens and a cat. The wind sounded like an airplane engine, he said. And the fire was running across hillsides against the wind.

“It looked like someone was throwing sand,” he said, “except it was fire.”