Thousands of residents were able to return home as the Saddleridge Fire reached 41 percent containment.

As thousands of people returned to their homes and firefighters appeared to get the upper hand on the 7,965-acre Saddleridge Fire in the Northern San Fernando Valley, questions turned to the events Thursday night that sparked the deadly blaze in Sylmar.

Los Angeles Fire Department officials are investigating reports that the blaze began at the base of an electrical transmission tower. Southern California Edison opted not to shut off the power lines during the Santa Ana wind storm in the Sylmar region that has endured three catastrophic wildfires within 11 years.

The Los Angeles Times interviewed a Sylmar couple that said they watched flames burn at the base of the transmission tower from their second story-window. They called the fire department, but by the time the first engines arrived, the flames had spread rapidly. Edison officials cautioned against a rush to judgement.

“We did not deenergize any power for the Saddleridge fire area,” said Edison spokeswoman Sally Jeun told the Times. “Determining the cause and origin of the fire is a lengthy process. Our priority right now is ensuring the safety of our customers, employees and first responders. SCE will fully cooperate with investigations.”

Investigators have not yet determined the cause of the fire, said Los Angeles Fire Department Chief Ralph Terrazas. City officials had been working with the Los Angeles Homeless Services Authority to clear homeless people out of fire-prone areas during the red-flag conditions that began Thursday, but he said he did not know whether there were any encampments near the flashpoint of the blaze.

The fire was first reported just after 9 p.m. Thursday off the westbound Foothill (210) Freeway near Yarnell Street and Saddle Ridge Road in Sylmar, and quickly spread due to wind-blown embers that jumped the Golden State (5) Freeway about 11:20 p.m., spreading the flames into Granada Hills and Porter Ranch.

Firefighters worked through the night Saturday to fill in gaps in containment lines and put out any hot spots in the Saddleridge Fire. At last report, the blaze was 41 percent contained by lines of cleared vegetation and had damaged or destroyed 32 structures and forced about 100,000 people temporarily from their homes in the areas of Sylmar, Granada Hills and Porter Ranch, fire officials said.

An unhealthful smoke advisory was issued through Monday morning by the South Coast Air Quality Management District for much of Los Angeles County including the Northern San Fernando Valley, Malibu, the San Gabriel Valley and Long Beach. People who can smell smoke or see ash are advised to remain indoors with windows and doors closed, and avoid vigorous physical activity. Officials said that winds were expected to push the smoke east into Pasadena.

A total of 21 structures were destroyed, according to the latest update by fire officials. That includes 16 single family residences, two multi- family residences, one commercial property and two minor structures. Eleven structures were damaged, including nine homes, one multi-family residence and one minor structure, fire officials said.

Caltrans reported Saturday evening the reopening of the southbound Golden State Freeway truck route, the southbound Antelope Valley Freeway to the southbound Golden State Freeway truck route and the northbound Golden State Freeway to the northbound Antelope Valley Freeway truck route.

Humidity levels increased to 20-25% by Saturday night and the winds died down to 3-5 miles per hour, National Weather Service Meteorologist Kristen Stewart said. A red flag warning expired at 6 p.m.

The man who died of a heart attack Friday morning in the Porter Ranch area while trying to protect his home from the fire was 54-year-old Aiman Elsabbagh, according to media reports. Terrazas said Elsabbagh was actually speaking to firefighters when he went into cardiac arrest, and he died at a hospital. According to reports from the scene, the father of two had been dousing the flames with his garden hose when he had the heart attack.

Two firefighters were reported injured as of Saturday evening. One was a minor eye injury, according to the LAFD. No details about the other injury were available.

The massive fire had prompted a mandatory evacuation order for all residents of Porter Ranch north of the Ronald Reagan (118) Freeway from Reseda Boulevard to DeSoto Avenue, residents of Granada Hills from Balboa Boulevard and north of Sesnon Boulevard to the Ventura County border, and the Oakridge Estates community north of the Foothill Freeway in Sylmar. In all, it affected roughly 23,000 homes — equating to about 100,000 people, authorities said. All evacuation orders were lifted Saturday.

Roughly 1,000 firefighters from LAFD, Los Angeles County Fire Department and Angeles National Forest were on the ground battling the flames, aided by water-dropping helicopters and fixed-wing aircraft dropping fire retardant.

Los Angeles Mayor Eric Garcetti, who cut short a trip to Copenhagen, Denmark due to the fire, and county Board of Supervisors chair Janice Hahn both signed local emergency declarations.

Gov. Gavin Newsom declared a state of emergency for Los Angeles and Riverside counties. The declarations free up local and state resources to aid in the firefighting effort.