Two firefighters were shot dead and a third wounded on an Idaho mountain when they were ambushed by a man who set the fire to lure them to the area, the local sheriff says.

The identities of two firefighters killed and one who was injured have been released after a gunman reportedly intentionally set a fire and opened fire on crews responding to the fire on Sunday afternoon.

An investigation is underway after authorities say a lone gunman started a fire and ambushed firefighters who responded to it in north-west Idaho on Sunday, allegedly shooting and killing two and seri