The eucalyptus branch hit the line, creating an electrical arc that sent sparks to the ground, investigators concluded. The power lines themselves continued to deliver electricity, but the sparks set out a fire that spread quickly amid the dangerous Santa Ana winds and dry brush.
"This was, simply put in plain parlance, an act of God," Mayor Eric Garcetti said.
The power lines were located near the 1800 block of North Sepulveda Boulevard to the west of the 405 Freeway and the fire began around 1:30 a.m. Monday.
Garcetti and other officials said the city had done brush clearance in the area over the summer beyond what was legally required. The tree where the branch came from was about 25 feet away from the power line and 20 feet up the hill.
The investigation was aided by dashcam video from a car driving through the Sepulveda Pass that caught what appeared to be an electrical arc explosion along the side of the road. Officials also found the charred eucalyptus branch in the area.
In the meantime, the firefight continued Tuesday. While the flames were mostly under control, firefighters remained on guard to prevent the blaze from rekindling ahead of a major Santa Ana wind event expected Tuesday night.
Firefighters worked overnight to spray homes in the Brentwood area and other neighborhoods to stamp out any existing embers. Many homes in Brentwood were seen red-tagged, charred and unrecognizable.
Fire officials said it's critical to get ahead of the coming Santa Ana wind event, which is expected to arrive in the Southland Tuesday night. Forecasters say this round of wind may be the strongest of the season so far.
At a Tuesday afternoon press conference, Mayor Eric Garcetti said the fire had increased slightly in size to 656 acres and was at 15% containment.
LAFD Chief Ralph Terrazas said crews are on alert ahead of the wind event.
"Our goal today will be to increase containment as much as possible. That is our primary objective," he said. "Embers are known to travel for miles...We are very concerned about tonight's wind event."
MORE: Time-lapse video shows Getty Fire flames spreading over hills for hours
The Getty Fire erupted at 1:30 a.m. Monday along the 405 Freeway in the Sepulveda Pass, prompting the mandatory evacuation of thousands of residential and commercial structures. The blaze has destroyed 12 homes and damaged five.
MORE: See full list and map of evacuation area, road closures due to the Getty Fire
"We know we're going to have a major wind event tonight at about 11 o'clock that's going to last until Thursday. We're doing everything we can to wrap our arms around this fire to be able to prevent a potential of those strong gusty Santa Ana winds, pushing this fire, rekindling a lot of the fire and blowing embers a mile to two miles down range," said Assistant Chief Jaime Moore with Los Angeles Fire.
PHOTOS: Getty Fire burns in West Los Angeles
Gov. Gavin Newsom spoke at a press conference Tuesday. "FEMA has been extraordinary. Trump administration has been extraordinarily supportive and helpful. Every request we've made has been granted. That support has come in real time," he said.
Garcetti announced he was signing an emergency declaration to bring more resources to help battle the blaze. During a press conference, Garcetti said the fire was not caused by an encampment or by someone without housing.
The mayor said investigators were making "good progress'' in determining the cause. They were looking at multiple possibilities.
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