"The man came down the trail carrying his son," said park visitor Shawn Ardaiz. "And it looked like he had a laceration on his neck and was bleeding pretty heavily all over."
Posted signs warn the hiking area is mountain lion habitat, and sightings are common. But attacks on humans are extremely rare.
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Investigators were told the cat attacked from behind and tried to drag the child away.
"It did end up taking him to a brushy area, and his parents did have to fight the animal off," said Fish and Wildlife warden Travis Jarrett.
The families had walked about a mile up a path known as the Zinfandel trail, and the mountain lion pounced soon after they had turned around to head back.
"We saw a small child, and they were wrapping his head," Julie DeWirst said after witnessing the aftermath.
DeWirst said the child was awake, calm and quiet while waiting for paramedics.
"That makes your heart just break, and to think the parents were there and saw it grab him; I can't even imagine going through that," DeWirst said.
At Valley Medical Center in San Jose, the boy was in fair condition Sunday night.
"Everything that could be done is being done, and he's improved already so that's a good sign," said spokeswoman Joy Alexiou.
"His parents are with him. They are concerned and attentive. He's getting all of their focus and he's getting great care."
As the youngster recovers, park rangers and wildlife wardens are working to find the mountain lion. They are bringing in a federal trapper with expertise in tracking the cats. When they find it, they will use DNA from it, as well as the victim's clothing, to make sure it's the right animal. It will be tested for rabies and euthanized.
Warden Foy said the mountain lion must be euthanized to protect public safety and because the animal stalked the group – even after attacking the boy and being scared off. Its tracks show it continued to follow them as they fled to safety.
"This is all extremely rare, and we need to find out what provoked the animal to attack," said chief park ranger Matt Anderson. "There's nothing this family could have done differently. It's an isolated incident, definitely, in our park system."