The Dog Rock Fire near Yosemite National Park ignited from sparks generated by hot metal fragments from a vehicle, according to park spokesperson Kari Cobb.
The sparks may have come trailer safety chains dragging on the pavement, vehicle brake parts generating a spark or other sources.
The 245-acre fire broke out early Tuesday afternoon between the park boundary and Arch Rock Entrance Station. It continues to burn along Highway 140, east of El Portal. The highway is closed.
Crews had 10 percent containment as of Thursday morning.
A Cal Fire air tanker assigned to the fire crashed Tuesday afternoon, killing 62-year-old pilot Geoffrey "Craig" Hunt.