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 and it wasn't immediately clear if there was one gunman or more, while urging the public to stay clear of the area.
"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.
The Canfield Mountain nature area that is popular with hikers near Coeur d'Alene, a city of 57,000 people located about 420km east of Seattle in Washington state.
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."
Norris said it appeared the sniper or snipers were hiding in the rugged terrain and using a high-powered rifle. He said he instructed deputies to fire back.
"I'm hoping that somebody has a clear shot and is able to neutralise, because they're not at this point in time showing any evidence of wanting to surrender," the sheriff said.
They were in a spot "with heavy brush and they are well prepared and blending in with their surroundings", he said.
"If these individuals are not neutralised quickly, this is going to be likely a multi-day operation," Norris said.
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.
Gun ownership is widespread in the US, where the country's constitution protects the rights of Americans to "keep and bear arms".
Deaths related to gun violence are common - 17,927 people were murdered by a gun in 2023 in the US, according to the most recent available data from the Centers for Disease Control and Prevention.
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