The Kootenai County Sheriff's department said a tactical response team found the dead man late on Sunday.
Officials did not release his name, nor did they say what kind of firearm was found.
Though the shelter-in-place order was lifted, the sheriff's office cautioned residents to be prepared because the fire was still burning.
Sheriff Bob Norris had earlier said law enforcement officers and firefighters were taking sniper fire and urged people to stay clear of the area around Canfield Mountain, a nature zone popular with hikers near Coeur d'Alene, about 420km east of Seattle.
"This evening, members of the SWAT team located a deceased male on Canfield Mountain. A firearm was found nearby," the sheriff's office said in a statement.
"At this time, the shelter in place is being lifted, however there is still an active wildfire on Canfield Mountain. Residents in the area are advised to be prepared and ready should further action need to be taken."
The sheriff said the shooter had used high-powered sporting rifles to fire rapidly at first responders, with law enforcement initially unsure of the number of perpetrators involved.
They were in a spot "with heavy brush and they are well prepared and blending in with their surroundings," Norris added.
Law enforcement is investigating whether the fire could have been intentionally set in order to lure first responders to the scene.
"FBI technical teams and tactical assets are currently on the scene providing support," FBI deputy director Dan Bongino wrote on X.
"It remains an active, and very dangerous scene."
Firefighters received the first call of a fire around 1:21pm on Sunday, Norris said, and about 40 minutes later, reports emerged that they were being shot at.
"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."
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.
They were in a spot "with heavy brush and they are well prepared and blending in with their surroundings", he said.
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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