MT LEMMON, Ariz.- The fire began Sunday afternoon and spread in a flash through dry grassland.
Now investigators think they know how the fire started.
Investigators say the fire began just above Mile Marker 4 near Molino Basin.
The Forest Service does not officially allow shooting on that piece of land but doesn't prohibit it either so people come there and fire rifles, pistols and shotguns at the rocky hillside.
Investigators think a shot, or a blast from a shotgun hit rocks, created a spark, and started the fire.
Pima Sheriff's Deputy Lee Hayes says it's not illegal to shoot there. It would be illegal if shooters were using something like Tannerite which explodes when a bullet hits it.
Deputy Hays says, “If you're shooting an accelerant or Tannerite, definitely a recklessness factor. If you're shooting steel anywhere in the desert. It doesn't matter whether it's windy or not. If we're shooting steel rounds, next to rocks, we're going to get spark spark creates fire."
The Molino Basin campground is just over the ridge from where the fire began.
Wally Crane is the campground host there. He saw the fire begin, called for help and cleared the campground.
"I just came back up here and I made sure I knew there was people on the trail up there on bikes and people walking and I went up there and I made sure I blocked off the area going towards up the mountain there on the Arizona Trail made him come into the base and where it was safer and get out quicker."
He's impressed and grateful about how fast firefighter attacked the fire but hopes people take a lesson in how a small spark can start a big blaze.
If authorities can find the shooter who started the fire it’s not clear if he or she will face any penalties.