The service said Wednesday the Mendocino Complex - twin fires that erupted in July in Lake County - scorched 720 square miles (1,865 square kilometers) of brush and timber north of San Francisco, destroyed 157 homes and killed a firefighter.
Officials says more than 400 firefighters remain in the area, doing repair work to prevent erosion and monitoring spots that continue to burn.
The blazes prompted the evacuation of thousands of people in Mendocino, Lake, Colusa and Glenn counties.
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The smaller blaze was fully contained on August 14 but a massive blaze continued to burn in the Mendocino National Forest.
Authorities are still investigating what ignited the blazes.
Get the latest on wildfires burning across California here.