APPLEGATE (AP) — Fire crews strengthened containment lines and put out hot spots Friday around a wildfire burning near a Northern California interstate that destroyed six homes and was threatening hundreds more.

The blaze along Interstate 80 about 40 miles northeast of Sacramento was holding steady at 420 acres while containment increased to 30 percent, state fire officials said.

The fire was threatening 1,000 structures around the Placer County community of Applegate, with many homes under mandatory evacuation orders.

Though residents may see less smoke in the area, there are still hazards, including downed power lines and trees, that are forcing officials to keep the evacuation orders in place, California Department of Forestry and Fire Protection spokeswoman JoAnn Cartoscelli said.

The cause of the blaze was under investigation, though authorities say they received reports of several car tires burning along the eastbound shoulder of the interstate as the blaze got underway Wednesday.

Meanwhile, officials said sparks from a vehicle at the edge of Yosemite National Park caused a wildfire there that led to the death of a firefighting air-tanker pilot.

Investigators had not determined what type of vehicle or what part may have caused the sparks, saying it could be the result of several things such as trailer safety chains dragging on the pavement, vehicle brake parts generating a spark or other sources.

The fire had burned 245 acres and was 10 percent contained.

Within hours of the blaze breaking out Tuesday, a state fire air tanker crashed into a canyon wall, killing 62-year-old pilot Geoffrey “Craig” Hunt of San Jose.

CalFire’s fleet of 22 tankers remained grounded while the cause of the crash was investigated.