The boy suffered injuries described as non-life threatening, and the park has been temporarily shut down by the Los Angeles County Department of Parks and Recreation.
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An investigation is being led by the California Department of Fish and Wildlife.
Capt. Patrick Foy of the California Department of Fish and Wildlife said the boy was walking up some stairs, and the mountain lion came from behind and bit him in the buttocks.
The boy's father was able to scare the mountain lion off.
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The park will be closed until further notice due to the "aggressive" mountain lion seen in the area, the Department of Parks and Recreation said in a news release.
The boy's bite wounds were swabbed for DNA to try and match it to a known cat. In the meantime, authorities are searching for the mountain lion.
ABC Los Angeles affiliate KABC's AIR7 HD was over the area as authorities attempted to lure the mountain lion out of the hills with a deer carcass.
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If the mountain lion is found and the DNA matches up with the boy's bite marks, there's a good chance the animal will have to be killed.
"Everything we do in every part of our mission is to better help wildlife and people coexist," Foy said. "Having a situation like this where wild animals start attacking, especially small children, that's a super difficult part of our job."
Residents in the Stevenson Ranch area have reported seeing a cougar in recent weeks and worried about their pets, but few thought it would go after a human.
"It still terrifies me because it could happen to me," said Santa Clarita resident Francisco Salas, who frequently visits the trails above the park. "It could happen to anybody out here."