Nearly 5,000 battle ‘monster’ King fire in Northern Calif.

A massive wildfire burning in a forest region east of Sacramento grew by 3,000 acres Friday, fire officials have reported.

The California Department of Forestry and Fire Protection and the U.S. Forest Service, which are jointly managing firefighting efforts in the area, said that nearly 5,000 firefighters are battling the approximately 80,000-acre King fire, believed to be arson.

Nearly 3,000 people have been evacuated from the area, and 21,000 structures remain threatened by the week-old blaze, which was 10% contained as of Saturday afternoon.

“This fire is a monster,” said Cal Fire spokeswoman Lynne Tolmachoff.

The King fire started Sept. 13, growing for several days and then exploding in a matter of hours Wednesday night from 27,930 acres to 71,000 acres.

Warm temperatures, low humidity and forests full of extremely dry trees--the result of serious drought conditions--had all contributed to the fire’s severity, Tolmachoff said.

On Saturday, ground crews worked to strengthen containment lines around the fire to limit further spread of the flames. Teams fanned out to assess damage to structures burned by the blaze.

Cal Fire and U.S. Forest Service crews also split the fire into two management zones in an effort to improve logistics and reduce travel time for fire crews to reach their assignments.

According to Cal Fire spokesman Johnny Miller, who was stationed at the agency’s fire command post in Placerville on Saturday, state crews were planning to manage firefighters working at the southern end of the fire while federal teams would work in the north.

Miller said that splitting a fire into zones in this manner was unusual.

“It’s very rare we’d have this many firefighters and this much complexity in a fire,” he said.

Earlier in the week, a Cal Fire unit chief said firefighters had dropped “world record” amounts of fire retardant on the flames. But Tolmachoff said that firefighting aircraft had not been able to continue that work Friday because of smoky conditions over the area. She said aircraft had been grounded on Saturday as well.

National Weather Service meteorologist Jason Clapp, who was on site at the blaze, said possible thunderstorms in the area late Saturday could pose a threat to firefighters, creating winds that could strengthen the fire.

Additionally, he said, meteorologists were watching closely to see if shifting weather patterns early next week would generate winds similar to those present last week when the fire spread quickly.

El Dorado County is updating information about the fire -- including evacuation information and highway closures -- on its website. Last week the county Sheriff’s Department arrested 37-year-old Wayne Huntsman on suspicion of starting the blaze.

Cal Fire’s Tolmachoff said that an investigation into the crime was ongoing, so local officials had not yet released details of how the fire may have been set.