CHICO, Calif. — California’s largest wildfire so far this year was 40% contained Wednesday after scorching more than 670 square miles (1,735 square kilometers).

The massive Park Fire was allegedly started by arson on July 24 in a wilderness park outside the Central Valley city of Chico and spread northward with astonishing speed in hot and dry conditions, becoming California’s fourth-largest wildfire on record.

“There was minimal fire activity through the night,” Cal Fire said in its morning summary. “Firefighters are currently working on mop-up and tactical patrol operations to work towards containment and to prepare communities to be safe for repopulations.”

Parkfire pic.twitter.com/e4nJiIRlP5

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A large portion of the fire is within Lassen National Forest, where officials said it was remaining inside its existing perimeter.

Weather has been more favorable recently, with increases in relative humidity, the moisture that can help suppress fire.

More than 5,400 firefighters remained assigned to the blaze.

Related Story: Cooler Weather Helps Firefighters Corral One-Third of Massive Park Fire

The fire primarily impacted two counties, Butte and Tehama, destroying 641 structures and damaging 52, according to Cal Fire.

An arson case is pending against a Chico man who was arrested after a burning car was seen being pushed into a gully, according to the Butte County district attorney’s office.

Other large fires currently burning in California have significant levels of containment, except for a blaze that erupted Aug. 9 in Six Rivers National Forest in the state’s northwest corner. The Boise Fire, named for a local creek, covered more than 11 square miles (28 square kilometers) Wednesday with zero containment.