Updated Friday, 11:25 p.m.

REDDING, Shasta County — An explosive wildfire tore through two small Northern California communities before reaching the city of Redding, killing a local firefighter and a bulldozer operator on the fire lines and injuring three members of a Marin County fire crew. At least 500 structures have been destroyed.

Cal Fire Chief Ken Pimlott said in a Friday afternoon media briefing that the Carr Fire, which was sparked Monday by a disabled vehicle on a highway northwest of Redding, "is not close to being done."

On Thursday evening, the fire rampaged through the communities of Keswick and Shasta and burned across the Redding city limits. The city, 200 miles north of San Francisco, is home to about 90,000 people.

Cal Fire updated its report on damage late Friday night. In addition to the 500 structures destroyed -- the count of residences was not immediately available -- the agency said 5,000 structures are threatened.

An Associated Press reporter toured parts of the devastated area during the day Friday and counted a total of 126 burned homes -- 66 in the Lake Redding Estates neighborhood and another 60 in nearby Lake Keswick Estates.

The blaze is "taking down everything in its path," said Scott McLean, a Cal Fire spokesman for the crews battling the blaze. By Friday evening, the fire had covered 48,312 acres -- 75 square miles -- and was just 5 percent contained.

Officials said that 38,000 people had been displaced from their homes. The evacuated areas include French Gulch, Summit City, Shasta Lake City and parts of Redding.

With the fire continuing to exhibit extreme behavior, officials urged residents in the rest of the city to be prepared to leave quickly if evacuation orders are issued.

The weather outlook offers no promise of a respite, with a red flag warning in place until Monday morning. The forecast conditions depict an almost impossible situation for the growing army of firefighters struggling to contain the fire in the mountains and hills west and north of Redding: 110-degree temperatures, humidity between 5 and 10 percent and winds gusting up to 30 mph.

Two firefighers have died in the fire so far.

Late Thursday, crews found the body of a contract bulldozer operator hired by Cal Fire to clear vegetation in the blaze's path, McLean said. The fire burned over the operator and his equipment.

Cal/OSHA said the operator worked for Robert Dominikus General Engineering. This incident marks the third fatality of a contractor in a California wildfire in the last two years.

Early Friday, Cal Fire confirmed that Fire Inspector Jeremy Stoke of the Redding Fire Department had also been killed. A Cal/OSHA spokesman said his agency was told he was killed because of a fatal vehicle accident on a public road.

The three firefighters who suffered burns were identified Friday as members of the Marin County Fire Department: Scott Pederson, 37, an engineer, and firefighters Tyler Barnes, 34, and Brian Cardoza, 26.

The department said the three were treated at Mercy Medical Center in Redding for burns to the ears, face and hands, the department's release said. One is receiving additional evaluation at the UC Davis Burn Center in Sacramento.

The department said the three were injured by a sudden blast of heat from burning vegetation.

Residents on the western side of Redding who hadn't been under evacuation orders on Thursday night were caught off guard and had to flee with little notice, causing miles-long traffic jams as flames turned the skies orange.

"When it hit, people were really scrambling," Cal Fire's McLean said. "There was not much of a warning."

Sharon Stapleton said she left after a police officer knocked on her door on Thursday.

She told the Record Searchlight newspaper in Redding that she and her husband grabbed a pillow, some shirts and pants, medicine, important documents and their two dogs, and left in their sport utility vehicle and their motor home.

Many firefighters turned their focus from the flames to getting people out alive.

"Really, we're in a life-saving mode right now in Redding," said Jonathan Cox, battalion chief with Cal Fire. "We're not fighting a fire. We're trying to move people out of the path of it because it is now deadly and it is now moving at speeds and in ways we have not seen before in this area."

Some residents drove to hotels or the homes of family members in safer parts of California, while other evacuees poured into a shelter just outside of town.

A reporter with KRCR-TV choked up as she reported live updates about the fire before the station had to go off the air later. Two news anchors told viewers that the building was being evacuated and urged residents to "be safe."

A hospital official said a total of eight people with fire-related injuries, including the three Marin firefighters, have been treated at the Mercy Medical Center. Hospital spokesman Mike Mangas said none of the injuries are serious.

Mangas said people are also arriving at the Mercy Medical Center with respiratory problems.

Firefighters tried in vain to build containment around the blaze Thursday but flames kept jumping their lines, Cal Fire's McLean said.

"It's just a heck of a fight," he said. "They're doing what they can do and they get pushed out in a lot of cases. We're fighting the fight right now."

He said the fire behavior was so erratic, there were tornadoes within the fire "tossing around equipment, blowing windows out of Cal Fire pickup trucks."

Brett Gouvea, incident commander of the crews battling the fire, urged residents to pay close attention to the blaze.

"This fire is extremely dangerous and moving with no regard for what's in its path," he said Thursday, with flames exploding around Whiskeytown Lake. An effort to save boats at a marina by untying them from moorings and pushing them to safety, wasn't swift enough to spare them all.

Dozens of charred, twisted and melted boats were among the losses at Oak Bottom Marina.

"The boat docks down there — all the way out in the water — 30 to 40 boats caught fire when the fire laid down on top of them last night and burned those up," said fire Chief Mike Hebrard of Cal Fire.

Lt. Col. Jonathan Shiroma with the California National Guard said 250 troops have been deployed to Redding to help fight the flames. They have two C130 planes on standby to aid Cal Fire's air attack on the fire.

Bill Sessa, a spokesman from the California Department of Corrections and Rehabilitation, said that almost 600 inmates have been deployed to the Carr Fire -- many of whom have been redirected from battling the Ferguson Fire, just west of Yosemite National Park.

"We're rotating about half of our inmates every day," said Sessa. "Some of them are getting 24 hours of rest while the other groups are out there working 24 hours, and then they rotate. So at this time of the year, it is literally nonstop."

This story contains reporting from The Associated Press