By Paul Vercammen and Amir Vera, CNN

(CNN) -- At least seven people were still missing in Shasta County, California, as shifting winds, dry fuel and steep drainages helped the monstrous Carr Fire engulf almost 100,000 acres by Sunday night, authorities said.

The fire has claimed six lives, including a firefighter and bulldozer operator working to extinguish the blaze.

Sixteen people had been reported missing, but nine of those have so far found safe, according to Shasta County Sheriff Tom Bosenko, who spoke at a Sunday news conference.

Redding Police Sgt. Todd Cogle said, "We're finding that there are a lot of communication issues" making it hard to locate residents.

"We spoke with several people who left their cell phones at their residences and in this day and age people don't remember people's phone numbers," Cogle said.

The fire, which started a week ago, is only 17% contained, according to the California Department of Forestry and Fire Protection, known as Cal Fire.

High temperatures, low humidity and increased winds are all in the forecast, setting the stage for more explosive fire behavior, Chris Harvey with the Cal Fire Incident Management Team said Sunday.

The National Weather Service also warned that forecast conditions would worsen the situation.

"A dangerous heat wave will continue from California to the Pacific Northwest early this week. Triple digit heat combined with dry humidity will only exacerbate the ongoing wildfire situation in California," it said on Twitter

The blaze doubled in size Saturday. Flames have destroyed at least 874 structures in the area, Cal Fire said.

Great-grandmother and great-grandchildren die

The erratic flames are blamed for six deaths, including 70-year-old Melody Bledsoe and her great-grandchildren, 4-year-old Emily Roberts and 5-year-old James Roberts.

The three had been missing since Thursday when their house was overcome by flames. Family members confirmed Saturday that they died.

"With a heavy heart we are sad to inform you all that Mel and the great-grandbabies were confirmed to be in the home," the family said on a GoFundMe page.

In those final minutes, unable to get to his wife and their great-grandchildren, Bledsoe was able to get through on the phone.

Ed Bledsoe, Melody's husband, broke into sobs as he described speaking to his family by phone as we was rushing to try save them.

"He just kept saying 'Grandpa, come and get me," Bledsoe said, referring to 5-year-old James. "'The fire's coming in the back door. C'mon Grandpa.' I said 'I'm right down the road.'"

Another person was found dead after a fire consumed a house, Bosenko said Sunday.

"This fire is scary to us. This is something we haven't seen before in the city," Redding Police Chief Roger Moore told reporters Friday night.

More than 3,300 fire personnel are battling the flames with more than 300 engines and 17 helicopters, Cal Fire reported.

The fire began Monday when a vehicle suffered mechanical failure, officials said. Firefighters had been making progress containing it until Thursday night, when it began to quickly spread.

Firefighter, bulldozer operator killed

At least two people died fighting the fire, and several others have been hurt.

Jeremy Stoke, a fire inspector with the Redding Fire Department, died Thursday, the Redding firefighters union said.

Stoke, who joined the Redding Fire Department in 2004, was killed while assisting with evacuations, the union said.

A private-hire bulldozer operator, who was not identified, also died Thursday while battling the fire.

Three Marin County firefighters were treated for burns to the ears, hands and face after they encountered a "sudden blast of heat" from burning vegetation. Engineer Scott Pederson and firefighters Tyler Barnes and Brian Cardoza have been released from a hospital, the fire department said Friday in a news release.

A fourth firefighter was being evaluated at the UC Davis Burn Center, the news release said.

Eight people were treated for respiratory and burn-related injuries at Mercy Medical Center in Redding, a hospital spokeswoman said Friday morning.

'I can't believe it's gone'

The fire forced many residents to gather belongings and escape in just minutes.

"We didn't think the fire was going to come here, so we didn't really take things out like everyone else that was scrambling at the last minute to get out when we saw fire on the ridge," Dominic Galvin said.

A resident of French Gulch, west of Redding, said she had two hours to evacuate, but some people had only 30 minutes.

"It's terrifying," Rachel Hines told CNN affiliate KRCR. "You're frightened a little bit because you don't know if you're going to come back to your house and the town is going to be different."

Chris Corona went to his parents' home to look for their cat, Jinx. The home was gone, but Jinx was there, safely hiding in a bush on a hillside untouched by the flames that killed all the other vegetation.

Corona wept as he thought of things they lost in the house.

"I can't believe it's gone. All those memories, childhood memories," he said. "Stuff that parents save, like stuff you built as a little kid for your mom. I'm just glad my mom got all the valuable stuff that she wanted out."

Looting has been reported in abandoned residences in Redding, and some arrests have been made, according to Police Chief Moore.

The California Office of Emergency Services said in a tweet on Saturday that the state was "getting help from other states to help fight wildfires." Those states with resources on the scene included Arizona, Idaho, Montana, Nevada, South Dakota, Wyoming and Colorado. Other states, including Florida, Maryland, Michigan, New Jersey, New Mexico and Oregon, have resources on the way, the tweet said.

Other fires burning in California

In addition to the Carr Fire, more than a dozen other large wildfires are burning throughout California, stretching resources.

The Cranston Fire near San Bernardino has burned more than 13,130 acres and is 57% contained.

A man accused of setting several fires in Riverside County, including the Cranston Fire, was arrested last Wednesday, authorities said.

The Ranch and River fires in Mendocino County have collectively scorched more than 30,000 acres, Cal Fire said.

The Ferguson Fire in Mariposa County has burned 54,481 acres and is 30% contained. The fire has prompted the closure of the most iconic areas of Yosemite National Park, which will reopen to all visitors August 3, officials said.

A Sequoia and Kings Canyon National Park firefighter, identified as Capt. Brian Hughes, died fighting the Ferguson Fire after he was struck and killed by a tree Sunday, according to a statement from the Sequoia and Kings Canyon National Parks.

"Hughes was with the Arrowhead [Interagency] Hotshots for four years and will be dearly missed by everyone at the parks," the statement read. "We grieve his loss."