Peri Van Brunt remained in custody but had not been charged with a crime as of Thursday morning, federal prosecutors said.

If criminal charges are filed against Van Brunt, she will be arraigned in U.S. District Court in Fresno, said Carl Faller, chief assistant U.S. attorney for Fresno.

Investigators believe Van Brunt accidentally set the now 57,000-acre (23,070 hectares) McNalley fire on Sunday afternoon near a resort in Johnsondale, about 130 miles (209 km) north of Los Angeles, then left the park.

The fire has blackened the underbrush-choked slopes of the Kern River valley, which bisects the Sequoia National Forest, and threatens groves of centuries-old sequoias, including General Sherman, the world's largest tree by volume at 274 feet (84 metres) tall and 36 feet (11 metres) in diameter.

The fast-moving blaze grew by 7,000 acres (2,833 hectares) overnight but by morning was 5 percent contained, said U.S. Fire Service spokeswoman Mary Grim.

More than 1,500 fire personnel battled 200-feet-tall sheets of flames that came within a half-mile of two famed stand of sequoias - Pack Saddle Grove and the Trail of 100 Giants - but changed direction before reaching the trees or about 200 homes that were threatened, Grim said.

"We've had some good luck with fire - it's not very active along western flank where residences and the giant sequoias are," Grim said. "We have been able to get some containment on the western side."

The fire torched a large portion of the Sequoia National Monument, which lies within the national forest and is home to groves of giant sequoia trees.

The majestic trees, which grow only in a narrow belt of the Sierra Nevada Mountains, grow to heights of more than 300 feet (91 meters) and can live longer than 3,200 years, according to forestry officials.

Although the giant trees withstood centuries of fires, forestry officials feared their thick bark cannot withstand the superheated mammoth blazes that have plagued the western United States this fire season as a result of years of drought.

Federal emergency management officials said on Wednesday that wildfires have blackened more than 6.7 million acres across the nation this fire season - twice the annual average in recent years.

The cost of fighting the McNalley blaze, which destroyed a resort in Johnsondale and forced the evacuations of about 1,000 campers and residents, topped $2.8 million on Wednesday, said Ashlee Schultz, a Sequoia National Forest spokeswoman.