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 seriously injuring another.

Kootenai County Sheriff Bob Norris told a news conference that the third firefighter was stable but "fighting for his life" in the Kootenai Health campus in Coeur d'Alene, about 30 miles east of Spokane, Washington.

Details were scarce on what was described as a "heinous act" that has shocked the local community.

"We do believe...that the suspect started the fire, and we do believe that it was an ambush, and it was intentional," Norris said. "This was a total ambush. These firefighters did not have a chance."