REDDING (AP) — A stubborn wildfire in Northern California that authorities say was sparked by exhaust from a truck was threatening dozens of additional homes on Tuesday.

The Bully Fire around the rural community of Igo in Shasta County was threatening more than 50 homes, up from about 15 a day earlier, California Department of Forestry and Fire Protection spokesman Daniel Berlant said. Some of those homes were under mandatory evacuation orders, although Berlant did not know exactly how many.

The fire, which began Friday and has destroyed eight homes and ten other structures, had burned through 10 square miles, or 6,400 acres, as of Tuesday morning. It was 20 percent contained.

The rugged terrain and dry, hot conditions have proven difficult for firefighters, seven of whom have suffered minor injuries.

“This fire is burning in really steep country,” Berlant said. “It’s definitely a hike to get back into the area. Sprained ankles and heat exhaustion are common.”

More than 2,100 firefighters were working on the blaze, up from about 1,800 on Monday. Cooler temperatures were expected later in the week, though Berlant said the wind could also kick up.

A 27-year-old Sacramento man was arrested Saturday and accused of recklessly causing the fire and with marijuana cultivation, both felonies, said the California Department of Forestry and Fire Protection.

Freddie Alexander Smoke III was delivering material to a pot site when the exhaust from his truck ignited dry grass, authorities said. The Shasta County district attorney has not yet reviewed the case, and it could not be determined if Smoke had a lawyer.