The Kootenai County Sheriff's Office said crews responded to a fire at Canfield Mountain around 1:30pm on Sunday and gunshots were reported about 30 minutes later.
Sheriff Bob Norris told reporters he didn't know if anyone else was shot.
"We don't know how many suspects are up there, and we don't know how many casualties there are," Norris said.
"We are actively taking fire sniper as we speak."
People are still coming off the mountain, the sheriff said, so it "would be safe to assume" that others were still up there.
Governor Brad Little said "multiple" firefighters were attacked.
"This is a heinous direct assault on our brave firefighters," Little said.
"I ask all Idahoans to pray for them and their families as we wait to learn more."
Kootenai County Sheriff Bob Norris said two firefighters had been killed.
Local news reports citing police scanners stated that multiple firefighters had been shot - but this could not be immediately confirmed by Reuters.
Authorities in western Idaho urged residents to take shelter.
The sheriff's office in neighbouring Soshone County said on Facebook authorities were "dealing with an active shooter situation where the shooter is still at large".
An alert by the Kootenai County Emergency Management Office asked people to avoid the area around Canfield Mountain Trailhead and Nettleton Gulch Road, about 6.5 km north of downtown Coeur d'Alene.
No additional details were provided.
Coeur d'Alene is a city of 55,000 residents near the border with Washington.
with reuters
