WATCH: The Yosemite serial killer claims his fourth victim: Part 7

WATCH: The Yosemite serial killer claims his fourth victim: Part 7

Transcript for The Yosemite serial killer claims his fourth victim: Part 7

There’s actually a road that connects from where the cedar lodge is into yosemite. It’s a back road that very few people know about. The end of the road is where Joie Armstrong was living, and that’s the road that Cary Stayner took that day. He drove his international baby blue scout up to a forest where he’d seen bigfoot. He went up there with some regularity. He had gotten out of his truck and he was looking around. He wasn’t out hunting for anybody, but an opportunity presented itself. Now, Cary Stayner is down here in this area. He sees up at the green cabin this petite blond girl. It’s almost the weekend, and Joie’s very excited. She’s got a trip planned to meet some friends. She’s just going in and out of her house, packing up her truck, getting ready to leave. Her Toyota Tacoma is parked here. The back hatch is open. He then approaches her, as she’s putting things in and out of her truck. You just wonder about the randomness of it all. What if she was in the cabin or had packed up a half hour earlier? What if she had left? It turned out to be a situation where evil truly meets opportunity. I was just over there throwing rocks in the creek and just happened to notice her walk out again and again. It seemed like she was alone. He comes closer and as he comes closer, she is what he thinks he wants. Something instantly changes with Cary Stayner, and he’s ready to kill again. Just like in the first murders, he goes back to his truck and he gets his murder kit. He gets out his backpack. He’s got a gun. He’s got duct tape. He’s got a knife. He’s standing somewhere in this vicinity. He’s talking to her and he’s making conversation. Here is a guy who, you know, was big and strong and athletic and has these movie star white teeth. He would’ve been just a little oddity. But as he’s talking to her about bigfoot, he’s trying to look behind her and look over her shoulder to see if anyone is in the house. And then, pretty quickly, it would have gone to flat-out terror. That’s when I pulled out the gun and put it to her head. She turned around and freaked out. I told her to go inside. He uses the gun to direct her to the back of the house and into this rear bedroom. He starts binding her with the duct tape he’s brought in his kit. She fights with everything she has. He barely was able to overcome her. She was very strong. And I don’t mean just a strong woman in the sense of emotionally. She was physically strong. This was supposed to be easy for Cary. This was, you know, his fantasy was to — that nobody’s supposed to fight back. And she did. She’s totally controlled, and then he takes her and guides her here. He picked her up and tossed her in the back seat and started driving away. The first sign Joie Ruth Armstrong was in trouble was when she didn’t meet up with her friend in Marin county on Wednesday. When Joie doesn’t show up, obviously her friends are bewildered and they’re frightened. Absolutely freaked out. Her friends called yosemite, and now the search was on for Joie armstro I received a page, and I called in. And they asked if I was available for a search. We’re covering a number of leads that are not confined to the park. That’s about all I can tell you at this point. As they came in to look, they could not find her. And they also found debris on the floor of the cabin. They found broken sunglasses. They found a red mechanic’s wrap. They were very concerned. You’re just looking for something that doesn’t fit. And then a few feet down the stream, I noticed what I thought was an inanimate object kind of bobbing in the water. And I went over and I saw that it was a person. To their shock and dismay, they saw that her head had been removed. This was just as grisly a scene as you could possibly imagine. It was incredible and horrible what had happened to Joie Armstrong. You know in your own personal life you’ve said many times before, “I can’t imagine the pain of losild.” You don’t believe it’s you. You don’t believe it’s her. You’re going, “No. No.” Cary Stayner left behind a load of evidence, and he knew it. Unlike the first three murders where he left virtually no evidence, he knew that he had left a very easy trail for investigators. About two hours north of yosemite is a nudist colony, which turned out to be the key to the whole thing. A car ride is about to happen. And during that car ride, the story of Steven Stayner and the story of Cary Stayner are going to intersect again in a way no one could have anticipated. ???

This transcript has been automatically generated and may not be 100% accurate.

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