California Bridge Fire updates: Newsom expands emergency due to 'extreme behavior'

The Bridge Fire straddling Los Angeles and San Bernardino counties was the largest of three major wildfires menacing Southern California on Thursday as federal assistance and the state National Guard were being deployed to help firefighters get the upper hand on the destructive blazes.

Some 2,500 structures are under threat from the Bridge Fire, according to the California Department of Forestry and Fire Protection (Cal Fire). Since starting on Sunday, the wildfire has consumed 51,167 acres of extremely dry vegetation mostly in steep, rugged terrain, officials said.

As of Thursday afternoon, the Bridge fire had destroyed 39 structures, including homes and businesses, and was 0% contained, according to Cal Fire.

The blaze -- the cause of which is still undetermined -- "continued to exhibit extreme fire behavior," although cooler weather conditions were helping fire crews battling to save homes, Cal Fire said. The fire grew overnight roughly 13,000 acres "mainly on the north and east flanks," officials said.

At least 13 structures were destroyed in the Wrightwood area, with power infrastructure also affected. The Mountain High West and East Ski resorts were both affected by fire activity, while 20 homes in Mount Baldy and six wilderness cabins were destroyed.

Cal Fire warned that the expanding inferno may cause "spot fires" as it reaches areas of "receptive fuels with little to no fire history."

The Bridge Fire is one of three large wildfires active in Southern California.

The Airport Fire

The Airport Fire continued to burn on Thursday across Orange and Riverside counties. The fire, which started on Monday, had consumed 23,140 acres as of Thursday and was 5% contained, according to Cal Fire.

The fire has left two civilians and 10 firefighters injured, according to Cal Fire.

At least 5,513 homes in Riverside County were under evacuation orders Thursday, and evacuation warnings have been issued for 9,581 homes. Some 53,000 Riverside County residents are affected by the fire.

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Gov. Gavin Newsom traveled to Southern California on Wednesday, proclaiming a state of emergency in Los Angeles, San Bernardino, Orange and Riverside counties in response to the Bridge and Airport fires, his office said in a statement.

The Airport Fire began around 1 p.m. PT on Tuesday, sparked by county public works crews working on a fire prevention project by trying to move boulders to prevent public access -- mostly by motorcyclists -- to an area of the canyon with a lot of dry vegetation that could ignite easily, officials told ABC Los Angeles station KABC.

The Line Fire

Newsom has also declared a state of emergency related to the Line Fire, which authorities alleged was deliberately set on Sept. 5. That wildfire -- east of Los Angeles in San Bernardino County -- was 18% contained on Thursday after burning 37,207 acres, Cal Fire said.

Over 100,000 people have been displaced from the affected area amid evacuation orders and warnings, with 65,600 structures threatened.

Justin Wayne Halstenberg, 34, was charged on Thursday with allegedly starting the Line Fire, officials said. Halstenberg is expected to be arraigned Friday on nine felony counts, including aggravated arson, arson causing great bodily injury and possession of materials and devices for arson.

San Bernardino County District Attorney Jason Anderson said at a news conference Thursday that three incendiary devices were found in the area where the fire ignited.

According to officials, the suspect allegedly tried to start the fire multiple times before succeeding.

Officials said they used automated license plate readers to identify a vehicle seen in the area of the Line Fire at the time it ignited and traced the vehicle to Halstenberg.

Newsom visited the Line Fire command post in Highland on Wednesday.

"California is deploying every available resource to combat these devastating fires, and we'll continue to work in lockstep with federal and local partners in this herculean effort," Newsom said.

ABC News' Marilyn Heck contributed to this report.