The so-called Valley Fire
raged in Napa and Sonoma counties near San Francisco in September last year.
In addition to the fatalities, including an elderly woman who couldn't get out of her house, four firefighters suffered serious burns that required extensive treatment.
It left behind a charred trail of destruction: 76,067 acres burned; 1,955 structures destroyed. At its peak, 4,000 firefighters tried to control the roaring flames that threatened to consume more homes.
The fire became the third most destructive wildfire in the state's recorded history, according to the California Department of Forestry and Fire Protection, which conducted the investigation.
How did it start?
The California Department of Forestry and Fire Protection, which goes by Cal Fire, said an investigation pointed to a faulty electrical connection as the cause.
The blaze started when a wire at a poorly connected hot tub overheated, melted and ignited dry brush nearby at a home in Cobb town.
"The ensuing investigation uncovered evidence that a residential electrical connection arced and ignited surrounding dry grass," Cal Fire said Wednesday in an extensive 173-page report.
An electrical engineer later determined that the connection was loose and not up to standards, according to the report.
"I found no records the electrical circuit was installed under the required building permit," deputy chief James Engel said in the report.
While it named the owner of the home where the hot tub was located, it's unclear whether he will face any charges.
"The homeowner admitted to installing the circuit to power a hot tub located on the side porch," the report said.
Cal Fire said it will submit the investigation report to the Lake County District Attorney's Office for review.
'It's changed forever'
The blaze displaced thousands of people, leaving some with blackened char where their homes once stood.
"It's such a beautiful area and it's changed forever," Bob Cummensky said at the time
as he peered over a devastated section of Middletown.
Craig Eve tried to look at it from a different perspective.
"It's a new beginning even though I've lost everything, and that's the way you have to do it. You have to have a positive attitude," he said last year.
Cal Fire officials said there was no evidence of arson in the Valley Fire.