Sparks from a hammer driving a metal stake into the ground ignited a 2018 blaze in Northern California that killed a firefighter and became the largest wildland fire in state history, officials said Thursday.

The blaze started July 17, 2018, in Mendocino County and quickly spread, aided by dry vegetation, strong winds and hot temperatures. It spread to Colusa, Glenn and Lake counties, the California Department of Forestry and Fire Protection said.

The fire burned a total of 640 square miles (1,660 square kilometers), much of it in the Mendocino National Forest, making it the largest wildland fire, or fire on undeveloped land, in state history. It also destroyed nearly 160 homes and killed a firefighter from Utah.

Cal Fire did not identify the person who ignited the blaze. It said no charges will be filed.

The Ranch fire was one of two side-by-side blazes dubbed the Mendocino Complex. The fires burned more than 700 square miles (1,813 square kilometers) of grass, brush and timber before they were contained. That's an area more than twice the size of New York City.

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This story has been corrected to say the fire in Mendocino County was the largest wildland fire in state history, not the largest fire in state history.