A wildfire with a ferocity never seen before by veteran California firefighters raced up and down canyons, instantly engulfing homes and forcing tens of thousands of people to flee, some running for their lives just ahead of the flames.

By Wednesday evening, a day after it ignited in San Bernardino County brush left bone dry by years of drought, the so-called Blue Cut Fire had blackened more than 25,000 acres in San Bernardino County and was four percent contained, firefighters said, revising an earlier estimate of more than 30,000 acres. The flames advanced despite the efforts of 1,300 firefighters.

Authorities could not immediately say how many homes had been destroyed, but they warned that the number will be large.

[LA GALLERY] Images From the Cajon Pass Wildfire

"There will be a lot of families that come home to nothing," San Bernardino County Fire Chief Mark Hartwig said after flying over a fire scene he described as "devastating."

More than 34,000 homes and some 82,000 people were under evacuation warnings as firefighters concentrated their efforts on saving homes in the mountain communities of Lytle Creek, Wrightwood and Phelan. They implored residents not to think twice if told to leave, but it appears many were staying.

Six firefighters were briefly trapped by flames during the fire's early hours, when occupants of a home refused to leave and the crew stayed to protect them.

Hundreds of cars packed with belongings and animals left the town. The air for miles around the blaze was filled with smoke.

The fast-moving blaze forced Interstate 15 to shut down in both directions, leaving drivers to find alternates routes through the region Wednesday morning as the wildfire continued to grow.

By Wednesday evening, the 215/15 northbound connector was to reopen by 10:15 p.m. After the connector was to reopen, the northbound Interstate 15 that travels through the Cajon Pass was also to be fully reopened.

At a morning news conference, authorities said the fire grew from 28 square miles late Tuesday to more than 46 square miles by Wednesday morning with no containment. Interstate 15 remains closed in both directions through the Cajon Pass, and State Route 138 is closed from State Route 2 to Interstate 15.

Multiple school districts in the area were closed due to poor air quality and tens of thousands of resident were evacuated.

"It hit hard. It hit fast," Hartwig said. "It hit with an intensity that we hadn't seen before."

[LA UGC PHOTOS] Blue Cut Wildfire Burns in Cajon Pass

Crews battled the destructive blaze through the night after it charred more than 28 square miles Tuesday alone, forcing the closure of the Cajon Pass, and burning structures with no end in sight. The devastating blaze came as a punishing summer heat wave swept across Southern California, which is enduring its fifth consecutive year of drought and in the midst of one of its worst-ever fire seasons.

On Tuesday night, Gov. Jerry Brown declared a state of emergency.

"This is not the time to mess around," said Battalion Chief Mark Peebles of the San Bernardino County Fire Department. "If you are asked to evacuate, please evacuate."

The Blue Cut Fire, named because it started near a trail called Blue Cut, erupted at 10:36 a.m. Tuesday in the Cajon Pass near Kenwood Avenue west of Interstate 15.

Less than 24 hours after the blaze began 60 miles east of Los Angeles, the fire command assembled a fleet of 10 air tankers, 15 helicopters and an army of 1,300 firefighters, many of them just off the lines of a wildfire that burned for 10 days just to the east. At a dawn briefing, half the firefighters raised their hands when an official asked how many had just come from the earlier blaze, part of a siege of wildland infernos up and down California this year.

Aerial Images: Fire Makes Path Toward Structures in Cajon Pass

The fire erupted in a landscape ready to burn after years of below-normal precipitation. The weather at the time was hot, dry and windy — conditions not expected to begin easing until late Thursday or Friday.

The pass is a major route for travel from the Los Angeles region to Las Vegas and also carries significant daily commuter traffic for high desert residents. The speed of the fire's spread astonished those in its path.

"This moved so fast," said Darren Dalton, 51, who along with his wife and son had to get out of his house in Wrightwood. "It went from 'Have you heard there's a fire?' to 'mandatory evacuation' before you could take it all in. This is a tight little community up here. Always in rally mode. Suddenly, it's a ghost town."

Missing Attachment A base camp has been set up at Glen Helen Regional Park to serve the hundreds of firefighters and helicopters fighting the Blue Cut Fire in San Bernardino County. Kim Baldonado reports for the NBC4 News at 5 on Wednesday, Aug. 17, 2016.

Big rigs were parked on both sides of the pass, a major highway and rail corridor through mountain ranges that separate Southern California's major population centers from the Mojave Desert to the north.

Road Closures:

South Bound 395 closed at Joshua

Highway 138 has been closed from Interstate 15 to Highway 2

State Route 2 closed from LA County Line to the 138

Hwy 138 closed between County Line to Highway173.

Old Cajon Blvd north of Devore Cutoff

Lytle Creek @ Glen Helen

Beekley Road from Phelan Road to the128

Hwy 38 to Lone Pine Canyon has been closed.

15 Northbound at I-215

15 Southbound at Ranchero

Detours

Eastbound I-10 to eastbound SR-62 to northbound SR-247 to westbound SR-18

Northbound I-5 to northbound SR-14 to eastbound SR-58 to I-15

SR-18 through Apple Valley and Lucerne Valley to SR-330 through Big Bear

School Closures

Adelanto Elementary School District

Apple Valley Unified School

Barstow Unified School District

Helendale School District

Hesperia Unified School District

Oro Grande School District

Silver Valley Unified School District

Snowline Joint Unified School District

Victor Elementary School District

Victor Valley Union High School District

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