A homeless man who shot dead two firemen after luring them into an ambush in Idaho had aspired to become a firefighter himself, his family has said.

Wess Val Roley, 20, used a high-powered rifle to kill Frank Harwood, 42, and John Morrison, 52, as they arrived to put out a 26-acre-wide wildfire he had started near a hiking trail on Canfield Mountain.

A third fireman, Dave Tysdal, was critically injured but is in a stable condition.