LOS ANGELES (AP) — A destructive fire that exploded on the edge of Los Angeles began beneath a high-voltage transmission tower owned by Southern California Edison, fire officials said Monday.

Los Angeles Fire Department arson investigators determined the origin of the Saddle Ridge Fire was beneath power lines on a dry, steep hillside above the city's Sylmar neighborhood, Capt. Erik Scott told The Associated Press. The cause remained under investigation.

The fire that started Thursday night burned nearly 8, 400 acres (13.1 square miles or 34 square kilometers), destroyed 17 structures and damaged dozens more. One man died of a heart attack during the fire in the Porter Ranch neighborhood, officials said.

The fire department had said Friday that a witness saw sparks or flames coming from a power line near where the fire was believed to have started. At least two people told LA TV stations that they saw fire near power lines above Saddle Ridge Road around the time the blaze broke out.

After several deadly blazes in the past two years have been blamed on trees and vegetation hitting power lines and other causes involving electrical equipment, utilities have been given authority to shut off power when fire risk is extremely high.

Just two days before the Los Angeles fire broke out, Pacific Gas & Electric Co. had cut power to nearly 2 million people in Northern California to prevent a repeat of catastrophic fires in that part of the state, including a blaze that destroyed the town of Paradise and killed 85 people.

SoCal Edison had warned it might cut power to close to 200,000 customers in communities throughout the region to prevent its equipment from sparking a wildfire. But it wasn't clear Monday if the fire originated in an area that could have had the power shut off pre-emptively.

SoCal Edison declined to comment on the fire's source. The utility reported to state regulators that its system was affected near the reported location and time of the fire, spokeswoman Susan Cox said.

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It filed the report Friday with the California Public Utilities Commission "out of an abundance of caution," Cox said. She did not have additional details such as the kind of system, how it was affected or what the effects were.

Electric utilities must file reports in four instances: when there is a fatality or injury that involves electric facilities, when there is damage to property over $50,000, when there is significant media coverage, and when there is a major outage to at least 10% of the service territory at once, said Terrie Prosper, a spokeswoman for the commission.

The report is confidential and not publicly available, Prosper said.

The fire erupted Thursday night and was rapidly spread by strong Santa Ana winds.

Residents of Saddle Ridge Road said they fled their homes as gusts drove the flames down the hillside and tossed embers into the neighborhood, lighting trees and setting bushes ablaze.

The neighborhood is bordered by powerlines strung between large towers.

Attorney Gerald Singleton, who has sued utilities for starting wildfires and has investigators looking into the blaze, said the fire's origin indicates it was probably caused by SoCal Edison's equipment.

"When you have a fire start that close to a piece of electrical equipment, you know, generally that's what causes it," Singleton said. "We certainly don't have any definitive evidence that it was their fault, but ... we're going to keep looking at it."

The fire was 44% contained on Monday.

To the east in Riverside County, a 1.5-square-mile (3.8-square-kilometer) fire that destroyed dozens of mobile homes and left two people dead last week was fully contained.

That fire ignited when a garbage truck driver dumped a load that had caught fire.