The woman who got lost in the wilderness and set a signal fire that exploded into a devastating wildfire will not face charges, Arizona’s federal prosecutor has said.

An explosive 500-square-mile wildfire has burned practically to the doorstep of the evacuated US mountain town of Show Low, Arizona, as crews fought to hold back the flames.

Two wildfires raging unchecked through paper-dry forest merged with a 50-mile-long fireball today, in a single blaze of about 300,000 acres that threatened to engulf an Arizona town.