Investigators have learned that sparks from a vehicle at the edge of Yosemite National Park were the cause of wildfire that led to the death of a firefighting air-tanker pilot this week.

Meanwhile, another series of Northern California blazes that destroyed five homes was threatened hundreds more near a major highway.

Fire investigators said Thursday that the blaze near Yosemite broke out because of hot metal fragments from a vehicle creating sparks, Yosemite said in a statement.

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."

They didn't immediately say what evidence led them to the cause of the fire that had burned 245 acres and was 10 percent contained.

Within a few 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.

"My dad died a hero," his daughter, Sarah Hunt Lauterbach, said in a statement released through the California Department of Forestry and Fire Protection. "There was not a day that went by that I didn't talk to my dad. He was my best friend."

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

CalFire spokesman Daniel Berlant said a decision to lift the safety stand-down on the department's own 22 S-2T air tankers will depend in part on preliminary results from the investigation into the crash.

"Once we determine that there is no clear mechanical issue with the entire fleet, then we will feel more comfortable having them in the air," Berlant said.

Meanwhile, the series of wildfires along Interstate 80 about 40 miles northeast of Sacramento were threatening 1,000 structures around the community of Applegate in Placer County, with many homes under mandatory evacuation orders.

Crews fighting those fires were relying on air tankers from the U.S. Forest Service and helicopters to drop fire retardant on the blaze, Berlant said.

It was not immediately clear what caused the fires, but at least two callers reported seeing several car tires burning along the eastbound shoulder of the interstate, California Highway Patrol officer Mike Martis said.

Dustin Andrews, a concrete fabricator from Sacramento, said he saw flames lapping at the roadside.

"We realized it was right up against the highway in four or five different areas," he said. "It happened so fast, but over such a wide area."

At Yosemite, the National Transportation Safety Board had two investigators at the site of the crash on the western edge of the park.

They were examining parts of the aircraft that they could reach, though the wildfire was limiting access to other parts of what was described as a nearly quarter-mile-long debris field, NTSB spokesman Keith Holloway said.