Meanwhile officials said a second firefighter died fighting a huge blaze to the south near Yosemite National Park.
Brian Hughes, 33, was struck by a tree and killed while working as part of a crew removing brush and other fuel near the so-called Ferguson fire's front lines, national parks officials said.
In Redding, officials stuck a hopeful tone for the first time in days.
"We're feeling a lot more optimistic today as we're starting to gain some ground rather than being in a defensive mode on this fire all the time," said Bret Gouvea, the California Department of Forestry and Fire Protection's incident commander on the blaze around Redding, a city about 230 miles north of San Francisco.
Mr Gouvea spoke at a news conference with fire and law enforcement officials.
Shasta County Sheriff Tom Bosenko said authorities found a sixth victim of the blaze at a home that was consumed by flames, though he declined to say where.
The victim's identity was not released.
The sheriff's department is also investigating seven missing persons reports, Mr Bosenko said.
Redding police have an additional 11 reports of missing people, though many of them may simply not have checked in with friends or family, said Redding police Sgt Todd Cogle.
The so-called Carr Fire that affected Redding - a city of about 92,000 people - was ignited by a vehicle problem on Monday about 10 miles west of the city.
On Thursday, it swept through the historic Gold Rush town of Shasta and nearby Keswick fuelled by gusty winds and dry vegetation. It then jumped the Sacramento River and took out subdivisions on the western edge of Redding.
The latest tally showed at least 517 structures destroyed and another 135 damaged, with the fire having consumed 139 square miles.
A count by The Associated Press found at least 300 of those structures were homes.
After days of fortifying the areas around Redding, fire crews were increasingly confident that the city would escape further damage. The fire had not grown inside the city limits since Saturday, Mr Gouvea said.
Some of the 38,000 people forced to evacuate said they were frustrated because they did not know whether their homes were standing or were destroyed.
Authorities had not reopened any evacuated neighbourhoods where fires raged due to safety and ongoing investigations and urged people to be patient, saying they would soon let residents back.
Fed up, on Sunday morning Tim Bollman hiked four miles on trails up steep terrain to check on the Redding home he built for his wife and two sons 13 years ago.
"There's not even anything to pick up," he said. "It's completely gone."
He took hundreds of photos, recorded video and texted his wife.
"It's the craziest thing you have ever seen," he said, his eyes filled with tears.
"It is what it is," he sighed, and then hiked off.
Keswick, a mountain town of about 450 people, was reduced to an ashy moonscape of blackened trees and smouldering rubble.
The San Bernardino County Fire Department was called in to tamp down smoking piles of debris that were scattered around the town amid downed electricity lines.
"What we're seeing here is an incomplete burn situation," Captain Doug Miles said as his crew used picks, shovels and rakes to open up piles that just days ago were family homes.
The flames laid waste to about 25 blocks, and the "mop up" work was likely to take days. He said his crew would be looking for anything salvageable, but there was little left standing.
The fatalities included two firefighters and a woman and her two great-grandchildren.
"My babies are dead," Sherry Bledsoe said through tears after she and family members met on Saturday with sheriff's deputies.
Her two children, five-year-old James Roberts and four-year-old Emily Roberts, were stranded with their great-grandmother, Melody Bledsoe, 70, when flames swept through the family's rural property Thursday on the outskirts of Redding.
The sixth victim, who was not identified, did not evacuate despite receiving an evacuation warning, Mr Bosenko said.
It is the largest fire burning in California, threatening more than 5,000 structures. The flames were just 5% contained, though Mr Gouvea said he expected that number to climb.
"We're here till the end, and we will get to an end, and we will bring some peace to this chaos," he said.
The firefighters killed in the blaze included Don Ray Smith, 81, of Pollock Pines, a bulldozer operator who was helping clear vegetation in the path of the wildfire. Redding Fire Inspector Jeremy Stoke was also killed, but details of his death were not released.