An erratic fire tearing a devastating path through one San Diego County community abruptly flared up Thursday with "explosive growth," destroying and threatening more homes in San Marcos even as its eastward spread into Escondido challenged fire crews battling five blazes across the region.

The Cocos Fire burned at least one home in San Marcos completely, and as it spread Thursday afternoon, another structure went up in flames on Phoenix Way.

Just hours later, a third structure turned into a fiery inferno near Coronado Hills and Seeforever drives when tall trees and a vehicle added fresh fuel to the fire. Firefighters said they let that structure burn because it was abandoned.

Explosions were heard as flames leapt toward more structures off Country Club Drive near Kauana Loa Drive in Escondido.

Escondido City officials confirmed Thursday evening that one home and two outbuildings were destroyed on Mount Whitney Drive in the unincorporated area of the county.

The fire jumped Coronado Hills Drive and approached several homes across the brush divide, moving faster than firefighters could over the hilly terrain and narrow, one-way roads, according to an NBC 7 News crew near the scene.

Flames could be seen marching up the hillside, burning eucalyptus trees in the backyards of homes along Via Del Caballo and Camino Hermoso. However, quick action from fire crews and shifting winds moved the flames away from the area.

From Bresa De Loma Drive, flames could be seen cresting the hill behind Harmony Grove Road, near structures. Fires are known to move quickly uphill, but a NBC 7 crew said this blaze is racing downhill just as fast -- as if pulled by gravity.

The flare-up came hours after fire officials identified the Cocos Fire as their number one priority among a spate of active brush fires that have ravaged the county over the last three days.

The fire, which was about 5 percent contained and burning 1,200 acres Thursday afternoon, has already destroyed at least five structures and forced the evacuation of Cal State San Marcos and at least 29,000 homes.

Evacuation notices were in effect for Questhaven, Harmony Grove, Elfin Forest, Coronado Hills, San Elijo Hills, Cal State San Marcos and Discovery Hills by noon Thursday.

Mandatory evacuations were ordered for Escondido residents living north and west of West Valley Parkway between Via Rancho Parkway and Highway 78 at 2 p.m.

To the south of the fire, residents were ordered to evacuate in the communities of Del Dios and Mt. Israel, north of Lake Hodges.

A new evacuation point has been set up at Escondido High School, at 1535 N. Broadway.

The fire was one of at least five still burning across the county Thursday, down from nine separate blazes that have scorched more than 9,000 acres. In all, the concurrent fires had consumed or damaged more than a dozen structures and caused tens of millions of dollars in damage.

The timing and proximity of the fires has fueled speculation that arson could be involved, though officials said it would be premature to comment on a cause in the early stages of the investigation. They noted that current weather conditions could cause even a small spark to ignite a brush fire.

"The grass out there is nothing but kindling for these fires, and we had winds, you know, very high speeds," San Diego County Sheriff Bill Gore said. "It only takes -- I was told by Cal Fire -- a few hundred degrees to ignite that grass."

Thursday afternoon, crews had to battle ever-changing wind patterns that began to pull behind the "plume-driven" fire, according to Cal Fire's Mike Mohler.

"So we actually have wind on this fire, but the fire is creating that wind. So you’ve seen the explosive fire growth this afternoon," Mohler said.

Crews have been on the fire line for more than 36 hours, so officials ordered in additional resources and started rotating the firefighters out.

Among those entering their third day of work was a Heartland Fire crew protecting homes on Cycad Drive, just off Coronado Hills Drive.

There, the fire got so close to one house that it melted the blinds inside the windows.

Firefighter Joe Howard said all the flames and heat went down both sides of the house.

“Initially when it came through, this was all red – fire on fire – so we had to wait for it to go through before we could actually get back here and start putting stuff out,” said Firefighter Joe Howard.

Overgrown brush on the property became an issue for the crew, and the flames destroyed a shed and a truck on the property.

Still, firefighters were able to save the house.

While a DC-10 Super Tanker was requested but not available, officials said in a morning briefing that the support ground crews received from San Diego helicopters with night drops overnight made a difference in controlling the fire.

Map of Wildfire Activity and Open Shelters

Complete List of School Closures | List of Areas Under Evacuation

Hot, dry conditions did nothing to help. Temperatures reached 94 degrees as of 10 a.m. but were likely to reach 101. Humidity was at 6 percent with winds at 3 mph. The extreme heat created additional challenges for firefighters, including those battling the fast-moving Tomahawk Fire charring 6,000 acres near Camp Pendleton.

Even with gains in parts of the county, including the complete containment of several of the fires, life remained on pause for many residents. School closures throughout San Diego kept more than 100,000 children home and thousands of homes and businesses remained under evacuation orders Thursday. Residents were urged to stay off the roads to make way for emergency vehicles. Schuler asked residents to respect the mandatory evacuation orders so crews can do their jobs.

“One of the challenges we faced yesterday is getting homeowners to leave and while we’re doing that we’re unable to fight the fire,” he said.

Gore echoed that messaging, saying 120 deputies have been stationed in San Marcos alone to protect the vacated properties. He advised residents to follow the directions of firefighters and stay out of the area as long as requested.

“We are watching your neighborhoods,” Gore said.

At its peak, officials said 250 people used the evacuation shelter at Mission Hills High School, and while about half had left by Thursday morning, organizers saw a new influx later that day as flare-ups continued.

The school will remain closed Friday so county officials can continue to use it as a shelter.

“It’s a crisis. We have to shift from education to taking care of folks. So that’s what we’re going to do,” said Mission Hills High School's Courtney Goode. “Tests can be made up and what not but lives are being heavily impacted right now so that needs to be our focus.”

Resources are available to all residents who may need recovery information through the county’s recovery website at sdcountyrecovery.com.

Check back for updates on this story.

