A body was found in the rubble in the area of the 400-acre Poinsettia Fire in Carlsbad on Thursday, officials said.
The badly burned body was found during a hot spot check in an encampment. It was the first death reported in the blazes that started this week.
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At least nine fires have charred more than 10,000 acres in Southern California as the region sizzled in triple-digit heat. As many as five fires were still burning as of Thursday night.
Two teens were arrested Thursday night in connection with starting at least two brush fires in the Escondido area. Police said 19-year-old Isaiah Silva of Escondido, and a 17-year-old juvenile were arrested at about 8:20 p.m. after concerned citizens called authorities about two individuals setting small fires.
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One of the larger fires burning in San Diego County suddenly flared Thursday afternoon due to a change in winds and burned close to homes as new winds arrived.
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The flare-up near the state university city of San Marcos occurred after a half-day lull in winds that firefighters had seized as an opportunity to make progress against flames.
"Firefighters did a tremendous job, it would all be gone if they weren't here," a San Marcos resident said.
Structure protection crews from Los Angeles were in San Marcos to help battle the blaze.
“We have strike teams from pretty much all of Southern California and they are working hard to make sure this fire doesn't get any bigger," an LAFD firefighter told NBC4. "(They) put their heart toward saving some of the structures and they're working hard to make sure that we don’t lose anymore."
Fires began erupting Tuesday amid high heat, extremely low humidity and gusty Santa Ana winds.
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Asked about the possibility of arson, county Sheriff Bill Gore said earlier Thursday that he wouldn't prejudge the investigations. He noted that sparks from vehicles can easily ignite brush in such dry conditions.
Ten military helicopters were being used to battle a blaze that grew to more than 9 square miles on the Marine Corps' Camp Pendleton. Despite its growth, the fire was 20 percent contained and was no longer considered a threat to communities.
Twelve other military helicopters were available to the county, where the biggest concern was a 1.25-square-mile blaze at the city of San Marcos.
That fire was 5 percent contained and thousands of people remained evacuated, but officials told a news conference they were beginning to assess repopulating areas.
The wildfires drove tens of thousands from their homes and shut down schools and amusement parks, including Legoland. The amusement park reopened Thursday.
Firefighters contended with temperatures approaching 100 degrees and gusty winds as they tried to contain flames fueled by brush and trees left brittle by drought.
Extremely high temperatures were occurring again Thursday, ranging in the high 90s to 100 in the northwestern area of the county where the fires burned.
The heat was so intense that records continued to be broken in Southern California and horse racing was canceled at Santa Anita Park in Arcadia, east of Los Angeles.
Officials said a Carlsbad-area blaze was 60 percent contained and had burned 400 acres. The wildfire destroyed an 18-unit condominium complex and four residences, Carlsbad Mayor Matt Hall said.
More than 20,000 evacuation notices were sent to residents Wednesday, and a California State University campus with nearly 10,000 students in the middle of final exams was shut down. Graduation ceremonies were canceled.
Tuzo Jerger was one of thousands told to evacuate because of the Carlsbad fire. The 66-year-old real estate broker packed files, a surfboard, golf clubs, clothes and photos and sought solace at a friend's hilltop house in nearby San Marcos, only to see a wildfire break out there and force thousands from their homes.
"I thought, 'Oh my God, it's going to come this way,'" Jerger said at a San Marcos restaurant where he found relief in a slice of pizza.
The blaze in the coastal city of Carlsbad, about 30 miles north of San Diego, was the most destructive of the fires so far.
Many schools across the county were closed Thursday. Officials expected some wouldn't reopen until next week.
Gov. Jerry Brown declared a state of emergency for San Diego County, which would free up special resources and funding for the firefight.
Drought conditions have made fire danger extremely high throughout much of California. Officials have encouraged residents in fire-prone areas to prepare evacuation plans and clear brush from near their homes.
Carlsbad's fire chief said the blazes were unprecedented in his 27-year firefighting career because they are so early in the year.
"This is May, this is unbelievable. This is something we should see in October," Chief Michael Davis said. "I haven't seen it this hot, this dry, this long in May."
The Poinsettia Fire in Carlsbad was 85 percent contained as of Friday morning, according to Carlsbad Fire Department officials.
Gadi Schwartz and Samia Khan contributed to this report.