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This imager provided by the Shasta County Sheriff's Department shows Freddie Alexander Smoke III who was arrested Saturday July 13, 2014, and accused of recklessly causing a fire and with marijuana cultivation, both felonies, according to the California Department of Fire and Forestry Protection. The wildfire has burned about 6 square miles of forested land in northern California. (AP Photo/ Shasta County Sheriff's Department)

REDDING, Calif. (AP) — Authorities believe a wildfire in Northern California was sparked by the exhaust from a truck driven by a man delivering supplies to an illegal marijuana plot.

Freddie Alexander Smoke III, 27, of Sacramento was arrested Saturday for investigation of recklessly causing a fire and illegally cultivating marijuana, according to the California Department of Fire and Forestry Protection.

The fire has burned 6 square miles, or 4,000 acres, of forested land in Shasta County and destroyed destroyed 18 structures, CalFire officials said. The fire, burning in steep terrain, was just 15 percent contained Sunday morning.

More than 1,700 firefighters, aided by aircraft, battled the blaze in hot, dry conditions.

The wildfire had prompted evacuations and road closures, but CalFire said all residents have been allowed to return home.

Still, the fire is threatening 15 homes and about 50 other structures.

In Solano County, a grass fire prompted evacuations in Fairfield and temporarily shut down Highway 12 in both directions. By 4 p.m. residents were allowed back to their homes and most lanes had reopened, officials said.

In Central California, firefighters made gains and contained 55 percent of a wildfire that burned more than 2 square miles (1,450 acres) of remote wilderness in the Sequoia National Forest, according to the U.S. Forest Service.

The forest where the fire started is south of and separate from the Sequoia National Park east of Fresno, which is famous for its giant Sequoia trees.