Florida, Tyler Hadley's house party is in full swing. 60 teenagers drinking and drugging with no parents in sight, Tyler's older brother Ryan away at college. One kid was walking around saying, "I... See More
Florida, Tyler Hadley's house party is in full swing. 60 teenagers drinking and drugging with no parents in sight, Tyler's older brother Ryan away at college. One kid was walking around saying, "I smell dead people." Other kids had seen some signs of blood, or what could have been blood. The house was a mess. It looked like it had been ransacked. Reporter: As the night wears down, Tyler is growing agitated. Do you recall him yelling at somebody trying to go back towards the back part of the house? Yes, I heard him just saying, "Get out of the hallway. I don't want anyone in the hallway." Reporter: How does he seem? He seems eager to tell me something, and he's -- wants to take me down the street and talk. Reporter: So, you guys walk out the house. Tell me what happens then. He says, "Mike, I killed my parents." And I said, "No, you didn't, Tyler. Shut up." And, he said, "Mike, look at the driveway, all the cars are there. My parents are dead. I killed 'em." Reporter: And then with partying teens still inside the house, Tyler tells Mike to look for the telltale signs -- blood by a computer desk where Mary Jo had been working. And by the master bedroom door -- but still Mike doesn't be I actually thought he went that far out to play a prank on me, that he actually put fake blood on the ground. Reporter: Really? You think he's playing a prank, at this point. Anything but what it is, you know. Reporter: The idea that teenagers are playing beer pong right here, while just steps away behind this door, his best friend's parents lay lifeless is too much for Mike to fathom. I took Tyler into the garage, and I said, "I want to see them dead, if you really did it. I don't believe you." Reporter: Tyler says he'll prove it after everyone leaves, but Mike doesn't wait. I walk around back, and I go check for myself. I come up to the door. The party's going on over here, and I turn the doorknob. Pushed it, it opened. I looked down, and I seen his father's leg against the door. Reporter: What was going through your mind when you saw his leg? My eyes popped up, and I said, "He's telling the truth, he did it. This is real." I'm in shock. I feel like there's no way I'm seeing this. Wake up. Reporter: Then, Mike Mandell does something as shocking as the party -- he doesn't leave. Now you'd think Mandell would call his parents or take his cell phone and surreptitiously call 911, no. Mandell continued to party hardy for several hours. Reporter: Why do you stay? It's -- it's a hard feeling to explain, you know. I knew this guy for so long. Something wants you to stay here right now. "You know, you need to talk to Tyler, and figure out as much stuff as you can." Reporter: How long do you stay? Couple hours. Reporter: How do you do that? How do you -- isn't it eating you up inside? It is eating me up inside. Even though you know he just killed two people, you don't see him as a killer. Reporter: You still see him as a friend, at that point? Yes. Reporter: Why not try to get Tyler out of the house immediately or call the cops? Because he could be a danger to everybody in that house, including you. I was still in shock. Reporter: Then, a bizarre moment between best friends. They take this selfie. I have to show you something here. Why did you take this picture? Because I knew it was going to be the last time I ever seen him. Reporter: So this was to remember him. Yeah. Reporter: Lot of people look at this picture and they say -- "Mike's callous. He doesn't care." What do you say to those people? Look at the picture. Am I smiling? Is Tyler smiling? When I look at that picture, he looks very worried to me. Reporter: And how do you look in this picture? I look like that this is the worst day in my life. I knew I was going to call the cops on him when I took that picture. Reporter: In shock and unable to keep it to himself, Mike tells another friend. He showed me the blood splatter on the baseboard. Okay, on the baseboard? Near his parents' bedroom. What did you do? I basically told him that I was getting the hell out of there. I was like, "I'm leaving. You can come with me." Reporter: Mike is still reluctant to leave his best friend, but when he finally does, Tyler is quick to notice. Tyler calls me on my cell phone. And he said, "Mike, where'd you go?" You know? "Party's still happening." And I said, "Tyler, I'm tired. It's 4:00 in the morning. I'm going to sleep." And he said, "Okay, man. Well, I'm having a party tomorrow. Are you going to come?" And I said, "Yeah. I'll be there." Reporter: This is a party Tyler never wants to end -- and at 4:40 A.M., he posts on Facebook -- "Party at my house again, hit me up" but Mike Mandel; won't be there. Immediately after I hang up the call, I call crime stoppers. We got a call from our call center. He's claiming he killed his parents -- Reporter: What were you thinking as you're making that call? It wasn't a good feeling. You know, I still knew that I had to do the right thing. Reporter: And the friend Mike told also calls police. The kids I spoke to who were at the party were shocked that they had been at a murder scene. But they also thought it was pretty cool that they'd been at this famous party. There had been a homicide. A 17-year-old young man had killed his parents with a hammer. Reporter: What was your first thought? First thought is, "No, 17-year-olds don't kill their parents with a hammer." Reporter: When detective Kristin Meyer arrived on the scene in the wee hours of the morning, remnants of the party are still apparent. We find beer bottles everywhere. They're all over the front yard. Reporter: Knock on the door. Lights go out. One officer gets cover and starts calling for backup. This officer hear the door opens. And there stands Tyler. Reporter: When we come back -- it's the question everyone wants answers. Why did Tyler snap was it a threat? A final ultimatum from his parents that pushed Tyler too far?
This transcript has been automatically generated and may not be 100% accurate.