Power poles that burned in the Thomas Fire in December 2017 dangle above Highway 150. (Sharon McNary/LAist)

The Ventura County Fire Department has determined that power lines were the cause of the Thomas Fire that ravaged the area in December 2017.

The fire started amid high winds that caused parallel power lines to slap into each other, creating an electrical arc that ignited flammable bits of molten aluminum on the ground. Those in turn, ignited dry vegetation, according to a report released today.

Southern California Edison owns the power lines. Being named liable for the start of the Thomas Fire, under California's laws, means the utility is responsible to pay for any damages. In this case, the cost could run into the billions.

The Thomas Fire began Dec. 4 and burned for almost 40 days. It threatened the cities of Santa Paula, Ventura, Ojai and Fillmore, as well as unincorporated areas of Ventura County, before moving into Santa Barbara.

Despite the efforts of nearly 9,000 emergency personnel, the Thomas Fire burned almost 282,000 acres — that's 440 square miles. It killed one civilian and one firefighter and also destroyed 1,343 structures, mostly homes.

One month later, during torrential rains, a mud flow originating with the Thomas Fire burn scar in the mountains above Montecito wiped out more homes and killed 23 people.

WHY DOES IT MATTER WHO CAUSED THE THOMAS FIRE?

It's all about who's responsible for the fire and who must pay the damages, which are likely to run into the billions of dollars. It's also about collecting evidence that might be used in a criminal case, if one is charged.

There are two known origin points of the Thomas Fire. The newly released report says Edison's power lines were responsible for the first ignition point, in Anlauf Canyon.

The utility had already said its equipment was "associated" with the second ignition point at the top of rural Koenigstein Road off Highway 150 near Santa Paula. However, that ignition is the topic of another, as-yet unreleased report.

HOW DID THE FIRE START?

The Thomas Fire was first reported at 6:23 p.m. Monday, Dec. 4, 2017. It was a day of extreme fire danger, low humidity and strong Santa Ana winds - 80 mile per hour gusts at times. However, at the outbreak of the fire, winds were in the 30 mph and under range.

The first fire started on a cattle ranch in Anlauf Canyon, above Steckel Park in the hills northeast of Santa Paula. There was a power interruption on Edison's system around 6:17 p.m. about an hour and a half after sundown.

The investigators conclude that the Thomas Fire was caused by wire slap. Two energized power lines hit together, casting molten aluminum particles onto dry vegetation, setting it on fire.

Edison pushes back on that conclusion. In a statement, it says it has evidence that the ignition at Anlauf Canyon started at least 12 minutes prior to the interruption on Edison's system and at least 15 minutes prior to the start time indicated by Ventura County Fire Department in its report.

Edison provided this evidence to the fire investigators, however, the report does not suggest this evidence was considered, the company said.

The second, separate fire was reported about an hour later off Koenigstein Road near Highway 150.

WHAT HAPPENS NOW?

There is a continuing investigation into whether any crimes were committed for which Edison or others could be charged. There is an enormous amount of loss of property and life in both the Thomas Fire and the Montecito mud disaster that followed.

Two people died during the fire - Virginia Pasola, 70, of Santa Paula whose car crashed along Wheeler Canyon while she was escaping, and Cal Fire Engineer Cory Iverson, 32, of El Cajon, who was caught in the fire.

The report refers to potential violations including manslaughter, unlawfully causing a fire, negligence, setting a forest fire, and failing to maintain the power system in a safe condition. There is an investigation underway by the state Attorney General into whether criminal charges could or should be brought, according to statements made in open court.

Then there's the civil litigation side. Now that the cause has been determined by fire authorities, it moves to the courts to establish liability and the amount of damages.

About 2,000 plaintiffs have already filed lawsuits naming Edison and others over their personal injuries and property losses in the two disasters. But more could come forward now that the official cause report is out. They have between two and three years to file cases.

This could potentially run into the billions of dollars for Southern California Edison ratepayers, and if negligence is shown, under California law, the burden to pay shifts to shareholders, and it comes out of profit.