go to your car with your keys. go alone. i have someone you love. if you don't you. >> tonight on "20/20," how did lights turn so dark for one of the golden couples? >> turn, turn, stop! >> emma is the sparkling, popular cheerleader. riley is the possessive football player. >> when things didn't go the way he wanted, sometimes he would claim he wanted to kill himself. >> this was a bad romance. >> she broke up with him. yes, finally.

>> but a mysterious man in black starts making things worse, much worse, for both of them. >> riley, i got kidnapped. >> he told me these people knocked him out, took his car, threw him in a van. >> she's being stalked by a man caught on video. >> out of the blue, someone just starts banging on her door. she's home alone, scared. >> so are her friends. when someone ends up dead. >> like, some romeo and juliet story. >> in the middle of the night, you heard a noise. >> tonight, we're taking you inside the investigation to catch the killer. the case in court. just hours ago. >> so you two now offer to help the police in a sting operation. >> the camera was on the tip of the key fob. you set it down, pointing right at the couch. >> i'm trusting you guys with my

life. >> shot in the dark. here's deborah roberts. ♪ >> reporter: it's friday night. the lights, the band. and everyone is decked out in red and black rooting for the central high bobcats of knoxville. >> knoxville is a rural community. there's a lot of mountain folks around here. >> reporter: though deep in the heart of the bible belt, here football is religion. >> fridays, everybody's excited at school to see how well we do. it's what our school revolves around. so it's exciting. >> reporter: but no one supports the bobcats quite like the cheerleaders. >> go bobcats! >> reporter: and in the fall of 2014 there was a new girl whose spirit was sparkling. 14-year-old freshman emma walker. >> emma really took cheerleading seriously.

she really loved doing it. you know, it was one of her passions. she loved leading and she loved football games. she loved just being part of crowd appeal. >> reporter: going into high school, being a freshman cheerleader, was that a big deal for her? >> she had done competitive gymnastics as a young child, and then competitive dance. when she tried out for cheerleader, she was the only freshman that made it. so she was very tickled with herself. >> reporter: at only 5'4", emma's a petite-sized powerhouse. but also a playful ball of energy. always moving to the beat. >> emma was very kind and warm hearted but she was goofy, too, at the same time. ♪ >> she could do the worm, but all of her other moves were kind of like, "okay, emma, you should probably stop now." >> reporter: that spunky cheerleader soon catches the eye of number 8.

central high's star player, riley gaul. he's like a speeding bullet on the field. was he the classic jock in high school? >> no, he was a little nerdy on the side. a little to himself kind of. from the outside looking in, you'd think he was just a normal guy. >> reporter: friends describe him as a jokester. raised by his mom and grandparents, riley seemingly a contradiction of high school cliches. the star athlete, also a top student who loves to play video games. what did you think right away of this new boy that she was smitten with? >> boy next door. he came in, very polite, very nice, in the beginning. >> i thought he was a very nice looking young man. well mannered. >> reporter: what were the rules about dating and going out? >> never would let them go out on a formal date. he could come over to the house, they would meet up after football games to eat, and

things like that. >> they had fun, they seemed like they really liked each other. we were happy for her that she found someone that she really liked. >> reporter: her social media splashed with pictures of the perfect pair. paddleboarding, cuddling, silly in their selfies. emma writing, "look how lucky i am." >> riley gaul and emma walker were just the sort of quintessential high school couple. you know, the cheerleader and the football player. >> reporter: what was the relationship like? >> at first it was just kind of normal, like, nothing out of the ordinary. he didn't really talk to us, her friends, a lot, but i was just like, oh, he's shy, like, new boyfriend, it just seemed normal. but then after a while was when we got kind of concerned. >> reporter: what concerned you? >> that he didn't really have any interest in getting to know her friends and he kind of

became controlling over her, what she did, her activities. >> he got more possessive and more clingy towards her, and, you know, wouldn't let her do certain things. >> reporter: riley and emma quickly become that other classic high school couple. the one that is always breaking up and getting back together again. and when they had fights, were they big fights? >> they were really dramatic, i would say, just because it was usually just like yelling at each other or just like harmful words towards each other. >> the relationship did not seem healthy at all. they argued fiercely and then they sort of made up fiercely. but it was always drama. always drama with those two. >> reporter: he would comment on what she wore, and tell her what not to wear? >> mm-hmm. yes. >> reporter: and what would you say to that? >> we're the decisions makers in that, not your boyfriend. >> when things got really intense and bad, he would wait outside her work for her to get off and he would just wait outside for hours. just wait.

>> reporter: did you all relate this to emma? did you feel concern for her, did she seem to feel concerned? >> i told her, and some of the other friends told her, we were just, like, you can do better, like, we don't like him. she just kind of brushed it off, she did her own thing. >> reporter: emma's friends say the football star becomes aggressive. take a look at some of his snapchats to his girlfriend. >> i hate you. i hate everything about you. you're the biggest -- i've ever come into contact with. >> riley didn't speak to her the way that a guy should talk to a girl. he was just disrespectful. >> reporter: one message in particular alarms her mom, jill. >> we on one occasion, saw one, i'll see your name in the obituary. >> reporter: he wrote that to her? >> he wrote that to her, and we questioned him about it and he said, i was just angry, and that's when it started to get many more red flags. >> reporter: emma's parents ban riley from the house and take away her cell phone, hoping to stop the obsessed teens from communicating.

it doesn't work. >> riley gave emma an ipod touch and she texted him through the wifi. >> reporter: for every nasty-gram he sends, there's a quick apology. >> emma, i'm sorry for however i act to you. i love you more than words can describe. >> reporter: did you attempt to break them up or advise her to break up? >> yes, several times. but as you do that with a teenager, the more you butt heads, the more she is going to think he is in the right. because he had a way of isolating her and making her think that he was the only one. >> reporter: next, a mysterious and frightening third person enters the picture. >> out of the blue, someone just starts banging on her door. >> i'm getting these really weird text messages, saying that riley's been kidnapped. ri(roger) being a good father is important to me so being diagnosed with advanced non-small cell lung cancer made me think of all the things that i wanted to teach my kids. (avo) another tru story with keytruda. (roger) my doctor said i could start on keytruda

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>> reporter: it's halloween, 2016, and it's not the decorations that have mark and jill walker feeling spooked. it's their high school daughter emma's two-year tumultuous relationship with boyfriend riley gaul, now an 18-year-old college football player who has cast a spell over her. at some point, you ground her. she's not allowed to see riley. and does that work? >> no. they would always find their way somehow to see each other. it became sneaky at that point. >> reporter: so jill brings down the hammer, confining her daughter to the house. only letting her out for school and cheerleading. shockingly, it appears to work. >> she did become like her old self again. she would come out of her room, eat dinner with us, and socialize with us. >> reporter: so you were seeing your daughter re-emerge? >> yes.

>> reporter: as for emma's relationship with riley, it's over. see her text message to her friends. >> well, we are done for good. >> she just came to the realization that she deserved better. like, this wasn't a good relationship, so she broke up with him. then we're all, like, "yes. finally, it's happening." we're like, "what we've been waiting for." >> reporter: but her boyfriend isn't taking the break-up well. over in his nearby college dorm, he takes some vicodin pills and washes them down with alcohol, an attempt to kill himself. >> one of the things that riley gaul was known for is when things would not go the way he wanted, he would sometimes claim he was gonna kill himself if they didn't reunite. >> reporter: how did you hear that he might have been suicidal? >> he would be off to the side, moping, saying things like, "oh, i just feel so depressed, i want to hurt myself," blah, blah, blah, blah. just things that he would just

say a lot as a cry for help, i guess in a way. >> reporter: it's friday night, and the bobcats have won a big playoff game. emma's allowed to join in the celebration at a friend's house. then, around 11:30 p.m., something strange begins to happen. zach greene is with emma that night. >> i walk in and emma was coming out of the bathroom and she's like, "zach, i'm getting these really weird text messages." "come outside alone if you don't want to see a loved one get hurt." >> i've got someone you love. if you don't comply, i will hurt them. >> reporter: was she spooked? >> yeah. she was definitely feeling really weird about it. i mean, it was a random number. >> reporter: creepy but emma thinks it's one of riley's friends playing a prank. then the messages become even more menacing. >> he's in the ditch beside her house. it's a shame you can all of a sudden not value someone else's life. >> we ended up going back outside. and we look, and we see a body laying face down. like, just a dark figure. >> reporter: through the glow of

the street light, a surprising sight, it's riley. >> then we finally get to him, and he's pulling up his head, kind of has this confused face on. emma's like, "why are you here?" he's like, "i don't know what happened, i don't know how i got here. i've been kidnapped. someone dropped me off here, i don't know what's happening, where am i? how'd i get here?" holding his head like he got hit upside the head kind of. it was just very weird. >> reporter: they are all left dumbfounded. and when you stepped out, where did you see him? >> he was kind of up there near kind of the shrub where those thin trees are, kind of laying down. part in the driveway maybe. >> reporter: lying down? >> yeah, like face down like this. >> reporter: and how did emma react? >> she was very upset, she was crying. she didn't really know what to think because, she's just like, "we just broke up, like, leave me alone." >> reporter: and if that's not bizarre enough, riley simply walks away and calls his close buddy, noah. >> he sounded like he'd been crying or he was sounding fatigued, i guess. he basically told me that these

people knocked him out, took his car and threw him in a van and he didn't know where he was. >> reporter: essentially kidnapped? >> yeah. >> reporter: did you believe him? >> not at all. but, what am i supposed to do? >> reporter: did anybody offer to call the police? >> we talked about it but he said no. he was just like, "no, no. no cops." >> reporter: the next morning, just hours after that supposed kidnapping, things go from strange to sinister. >> emma is supposed to go meet her mom and while she's home, out of the blue, someone just starts banging on her door. all she can see is this person. she's not opening the door. she's home alone. she's scared. >> reporter: emma texting her friends. >> i'm home alone and somebody in all black walked down my street and came to my door and rang the doorbell over and over again. i thought i was going to die. >> emma's obviously shook. and who do you think she calls? she calls riley. >> i'm shaking and crying. i hate you but i need you right

now. >> i'm coming. i'm speeding, just give me a minute. >> she was supposed to come meet me and she didn't show. i had an intuition, something's going on, so i drove to home, and riley's car was sitting out front of the house, and they were in the driveway. >> reporter: what was your first thought? >> i thought, you're kidding me. he knows he's not allowed here. i said, "you know you're not allowed, you need to leave." and he did leave. >> reporter: were you worried? >> yes, i was worried, yes. i tried to calm her down and take her mind off of it. >> reporter: but she's thinking, what, that some stalker, or maybe some burglar is trying to get into the house? >> i think. >> reporter: when we come back, an unimaginable discovery. >> where is your emergency? >> she's not responsive. >> reporter: inside the walker home. >> they were wrapping tape around our house. >> like some romeo and juliet story was my thought. >> reporter: stay with us. the fact is, there are over ninety-six hundred roads named "park" in the u.s.

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>> reporter: it's now sunday and emma walker is still on edge after a disturbing weekend. her ex-boyfriend claiming to have been kidnapped. and a mysterious man in black who frightened her just 24 hours earlier, banging on her door. were you worried? >> i was worried and we were watching her. i mean, sunday we followed her to work, followed her back home, to make sure she was safe. >> she said, i'm craving ice cream. so we drove to sonic, we had a nice cold sonic blast.

and i wished her good night, i love you, see you tomorrow. i went to bed, she stayed up. >> reporter: and how did she seem? >> she seemed very happy, very happy and relaxed. more of a -- like a weight had been lifted off her shoulder. >> reporter: you spoke with her on that sunday. >> we just talked about homework, we procrastinated over the weekend, just getting it done. >> reporter: did she seem nervous, upset? >> not when she was texting me. at this point i was like, okay, like, this is it, like, they've broke up for good, like, she's getting back to normal, riley's out of the picture. it just all seemed normal. >> reporter: it had been a crazy weekend but emma finally feels calm by the time she goes to bed just after midnight. by 6:00 a.m., it's time for school. >> i went in there to wake her up, which it's usually very easy to wake her up. and said her name, didn't hear anything.

bumped her leg, didn't hear anything. and then looked at her face and realized, and checked for a pulse and couldn't find anything. i don't remember a whole lot from that moment. i know i called 911. >> where is your emergency? >> i'm jill walker. >> what's going on there? >> i just tried to wake my daughter for school. and she has no pulse. >> how old is she? >> she's -- she's 16. >> you said that she's nonresponsive? >> yeah. >> the call came out as a suicide, it was about 30 minutes before shift was over. >> reporter: officer nikki bules is the lead forensic technician on the case. >> when i first got there, i started my photographs on the outside of the residence. i walked in, photographed the interior of the residence. photographed the bedroom. >> reporter: a typical teen girl's bedroom. messy with clothes and beauty products strewn about. pictures and cheer trophies on

the wall. but immediately officer bules notices something unusual. >> there was a hole in the wall. it appeared to be a bullet hole. at that point, i knew that it probably was not a suicide. >> it's been upgraded from a suicide to a homicide. >> reporter: lead detective allen merritt rushes to the walker home. he takes me back to the scene. >> so i'm standing outside here. the bullet hole is just about shoulder high there in the wall. >> reporter: a big hole? >> no, it's just a small bullet hole. about the size of a ballpoint ink pen. roughly about this area here i locate the spent shell casing, >> reporter: on the opposite side of the fence, another big discovery. a second shell casing? >> yes, ma'am. a second shell casing. so now i've got a second sell casing.

why is it here? where did it come from? >> so, another hole right there? >> yeah. >> reporter: 16-year-old emma walker, the vibrant cheerleader, now dead of a gunshot wound to the head. two bullets fired into her bedroom. one striking emma behind her left ear. the second lodged in her pillow. the shooter must have known she was sleeping behind that wall. >> once the detective got there, we were asked to leave the residence. they were wrapping tape around our house, and walking around outside, so obviously it had turned into a crime scene at that point. but we still had no idea what had happened to her. >> reporter: you talked to her did you ask them if they thought there was somebody who could be out to hurt her? >> we spoke to the family, we

spoke to her friends. everybody kept giving us the name riley gaul. because of their relationship, because they had seen the way that riley had treated her, the way he'd talk to her. >> reporter: so the knox county sheriff's department pays him a visit. >> he didn't say a whole lot. >> reporter: riley takes to social media to publically mourn his lost love. "rest easy now, sweetheart. i love you forever and always." >> what stood out to me from these tweets immediately was the repetitive nature of him saying, "i love you. i love you. i love you." if you just saw that, if you just saw that, you would think that this was an ex-boyfriend who just lost his first love. >> reporter: emma's friends are devastated. >> i was confused. all i knew is she was dead and there was nothing i could do. >> it didn't register in my mind. no one would expect our emma to be dead. >> reporter: a town in mourning, honoring emma with a candlelight vigil. >> students will be gathering at central high school tonight to remember a classmate who recently died.

>> reporter: her fellow cheerleaders releasing balloons in her memory during the next football game. >> big round of applause for emma walker, ladies and gentlemen. >> reporter: but riley's circle of friends are now worried about his well-being. they know he's previously attempted suicide. alex mccarty tells detectives about a secret riley shared with him saturday, just before emma died. >> he ended up telling me like that night, that he was so fearful for his life that he had stolen his grandfather's gun and he showed it to me. >> reporter: he showed you his grandfather's gun? >> yes, and i held it in my hand that night. >> reporter: so he's been acting strangely, and then he shows you a gun. how worried were you about him? >> i was very worried. he reassured me over and over again that he was the farthest thing from suicidal. he was just so scared of these people who were out to get him, were out to get emma. >> reporter: riley insists he needs the gun to protect himself and emma from those supposed kidnappers. and the man in black pounding at emma's door. what did you hear from the friends about what struck them

as odd? >> a person dressed all in black, head to toe, even sunglasses, a hoodie on, face covered, goes to emma's house, tries to get in, scares her. >> reporter: did you think maybe there was some other suspect out there, who had been stalking her? >> sure. >> reporter: now investigators are on the hunt for security cameras, hoping the killer might have been caught on tape. >> we had the exits from this residence. we saw cars going by, but we couldn't identify them. the cameras didn't have enough resolution. >> reporter: but one camera, just blocks away, does capture a shadowy figure. could it be the man in black? is this the killer? stay with us. i was getting all these leaves and i was going back generation after generation. you start to see documents and you see signatures of people that you've never met. i mean, you don't know these people, but you feel like you do. you get connected to them. i wish that i could get into a time machine and go back 100 years, 200 years and just meet these people.

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a shock for knoxville and a knox county high school, a cheerleader shot and killed. >> a knoxville teen found in her home dead. >> an investigation right now under way this afternoon in the death of a knox county teen. >> reporter: on social media, riley gaul is a picture perfect grieving boyfriend, citing bible verses to his lost love. >> this is from the gun case, from the grandfather. >> reporter: but police aren't buying his digital tears. he's now a person of interest in emma walker's death. just 16 hours after her body is discovered, the college football standout is running into a different kind of defender. >> you have the right to remain silent. >> reporter: detectives with the knox county sheriff department, on the offensive, wanting to know about the weekend he spent at his friend noah's house. >> who all's at noah's when you get there? >> it was just me and him, and

then, i think it was just me and him that night. i don't remember anybody else showing up. >> and you said you spent the night? >> i'm pretty sure i did, yes. actually, yes, i'm positive i did. i did spend the night. >> you can't remember if you spent the night at noah's or stayed at your grandparents'? >> reporter: did that strike you as strange that he couldn't remember if he spent the night here or what time he went there? >> yeah. we're talking about, you know, what'd you do yesterday, and not what you did two years ago. his answers were vague. >> reporter: vague and detached. throughout the nearly two-hour police interrogation, riley never utters emma's name, referring to her as the girl. >> the girl, she texted me. >> which girl? >> the one that passed away. >> oh, okay. what's her name? >> emma. >> okay. when i first met him at his grandfather's, i thought he might've been a grieving boyfriend. when we got into the interview room and sat down, i felt like there was a dark side. he didn't have a whole lot of passion or concern. >> reporter: riley tells detectives he'd been trying to speak with emma all weekend.

but she wouldn't engage. even blocking his number. >> but she said if i would help her write her paper, she would talk to me. and i used one of my friends' phones on campus to call her. >> tell me about this phone call you had with emma. >> i was crying, and i was trying to get her to reason with me on getting back together with me. and she just kept saying no. she said she didn't care about the relationship anymore. she said, "you need to stop. you're crazy. you need to get a life," hung up the phone, blocked me. >> reporter: then riley offers police his alibi. saying after hanging up with emma, he went to his grandparents' house, then drove back to his dorm, where he breaks down emotionally. >> i just sat there for about two, three hours, and just wept, just looking at the pictures of us and stuff like that. >> reporter: but where are those tears now? >> in my opinion he was emotionless. his interview was probably one

of the most disconnected, it almost seemed rehearsed, deliberate. you have no idea where your grandfather's gun is at? >> no, sir. >> okay. and you do not own a handgun, correct? >> no, sir. >> reporter: you began to ask him about the gun. >> he did not have the gun, didn't know where the gun was. and basically didn't have anything to do with it. >> reporter: now, you had already talked to friends of his who had told you that he had this gun. >> yes, ma'am. what if i told you someone told us that they saw you with a gun? what would you think about that? >> i would wonder who said that and where they saw me with a gun. >> alex mccarty said that you showed him a handgun. where is the gun? >> i do not know. >> you understand that for us, alex has no reason to lie about something like that. >> yeah, but i'm telling you i don't know where it's at. >> he said that you showed him the gun. you told him that you'd had it, and you told him that you got it from your granddad. >> i don't have the gun.

i don't know why he would say that. >> i knew immediately within minutes of sitting down to talk to him, riley gaul wasn't going to confess to anything. he wasn't going to do it. >> reporter: so detectives then turn up the heat, pressing riley about a strange request he made of his buddy noah. >> do you ever remember having a conversation with noah about getting fingerprints off a gun? how to get fingerprints off a gun? >> no. >> he asked if i knew how to get fingerprints off a gun. >> reporter: how to get fingerprints off a gun? >> mm-hmm. he said he was asking for his roommate. i told him, obviously not, and not to ever ask me anything like that again. and he said, "i know, i know, it was for my roommate. i thought it was weird." >> reporter: it finally dawns on riley, he's in serious trouble. >> am i a suspect in her passing? >> should you be? >> i just feel like i'm being badgered with questions. i hope to god i'm not a suspect in her death. >> did i say you were? >> i hope you don't think it,

because i wouldn't hurt that girl. >> did you shoot into emma's house? >> no, sir. >> reporter: and just like that, the interview is over. riley's head in his hands. but when he leaves the station, the star receiver goes on the defensive, texting his pal, alex. >> why did you tell him about the gun? they think i shot her because of it. >> i told him, he had no reason to withhold the weapon, if he had nothing to hide and that if anything, it could prove that he was innocent. >> reporter: alex says he can't take the lying anymore and calls out his calculating friend. >> i asked him, just outright, like, "what happened to the gun that you showed me?" and he said, "i returned it to my grandpa. you don't have to worry about it." and his mom walks out and begins questioning riley in front of me about the whereabouts of the gun. >> reporter: so, what are you making of this? >> he bold face lied to me. >> reporter: in texts, riley insists he didn't lie, asking his friends not to speak to police anymore.

>> i didn't [ bleep ] kill her. and i'm probably going to jail. >> reporter: what's his demeanor like? >> he's frantic about getting caught. >> he was on edge. >> and he was like, "the only thing we can do is i'll just have to get rid of it. i have to get rid of the gun in order for me not to get pinned for something that i didn't do, in order for me not to go to jail for something that i didn't do, i have to get rid of this weapon." that's when i knew, like, he did it. >> reporter: next, riley has a plan involving alex and noah. >> tell them you were on lsd, you were drunk. >> reporter: but what he doesn't know, they have a plan of their own. so you two now offer to help the police in a sting operation? the trap to catch a killer. when "20/20" returns. than rheumatoid arthritis. before you and your rheumatologist move to another treatment, ask if xeljanz xr is right for you. xeljanz xr is a once-daily pill for adults with moderate to severe ra for whom methotrexate did not work well enough. it can reduce pain, swelling and further joint damage, even without methotrexate. xeljanz xr can lower your ability to fight infections,

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>> reporter: engineering student

noah walton and buddy alex mccarty are convinced their longtime friend riley gaul killed his high school sweetheart emma walker. he said he wanted to go and throw the gun away, >> yeah, i just knew that if there was a chance he could get rid of the murder weapon. >> that you should stop it. >> i had to stop it. >> reporter: the two teens work with police to lay a trap. >> if they could help us get what they believed to be the murder weapon back. huge, huge piece of information. >> reporter: that missing gun, the key piece of evidence. authorities suspect the shell casings found at the scene came from riley's grandfather's missing 9-millimeter glock. they wire up alex and noah with microphones, a transmitter, so they can listen in, and a video camera hidden in a key fob, like this one. so you two now offer to help the police in a sting operation. were you worried about that? >> i was just fearful that maybe justice couldn't have been done if we made the wrong move.

>> the camera was right on the tip of the key fob. i set it down pointing right at the couch. >> reporter: it's now tuesday, the day after emma's death. the guys invite riley to come over to noah's house to play video games. >> i don't know who shot emma. i put that on my life. i wouldn't lie to you right here, right now. i don't know who did. >> i believe you, man. >> i would never hurt her a day in my life. i had that gun because i was scared about what happened. i know you don't believe me, you probably don't either, whatever. >> reporter: at first riley seems unsure whether to trust his friends, yet he has a plan and begins writing a script for them to throw police off his scent. >> tell them you were on lsd, you were drunk and you were high, your mind was altered, whatever statement you give them wasn't a straightforward answer. >> he basically wanted to nullify all our previous statements by saying we were under the influence, and we didn't know what we were talking about. >> reporter: what do you say to

that? >> i said okay. i didn't do it, but i was like, "okay, sure, bro, my bad. i didn't know i was going to get you in trouble." >> reporter: so you're playing him at this point? >> yeah. >> but i'm trusting you guys, like, with my life because, i mean, this is 70 years in jail if i get convicted of something i didn't do. and, are you guys, are you busy? right now. like, are you about to do anything? well, can we go to the bluffs? because i need to get rid of the gun. >> at the bluffs? >> i'm gonna throw it in the water. they'll never find it in the river. >> reporter: the bluffs is a wooded area down by the tenneesee river where teens often hang out to drink and get high. >> i want to get rid of it. well, do you guys want to go right now? >> reporter: and just like that, the trio jumps in alex's car. the sting is on. >> hey, we've got to go up to seth's house first. >> yeah. >> he tells us that he doesn't have the gun on him and we have to go pick it up from his

stepfather's house. he said he hid it down in the basement. me and noah wait in the car. >> he didn't want us to come in. >> he comes back with this trash bag. >> success? a bag of trash? >> i just threw it in there. i don't know what's in there. >> reporter: this is actual audio of the guys inside the car. >> i just want to throw it and be done. >> reporter: are you anxious, like, you want to see it right away? or do you know that you've got to pace yourself? what are you thinking? >> we tried a little bit. >> we did try to coax it. >> "dang, is it in there? let me see, it's cool. i don't see guns a lot." >> reporter: and he doesn't buy it. >> no, he doesn't. >> he actually was kind of weird about it. >> reporter: but alex and noah try to keep things as casual as possible. telling jokes, singing along to the radio. ♪ and going on a run for their favorite fast food. but what riley doesn't know, lieutenant steve sanders is in an unmarked suv tailing the boys. he takes me along the route.

early on you can hear the audio and then suddenly you lose it. what are you all thinking? >> it's kind of a, "oh, no. what do we do now?" it's a distance thing. >> reporter: fortunately, three undercover vehicles are riding alongside them. they communicate with noah and alex via group text. with the tension mounting, the guys make their way up to this apartment complex. >> they parked here and they sat in the vehicle for, i'd say, probably five or ten minutes or so, talking. >> he was putting on white gloves and waiting and everything. he was very meticulous about how he got everything out. then we just kept waiting and kept waiting. >> reporter: are you texting at that point, the police? >> i told them when we got the bag. i told them we didn't see it yet, but he had a bag full of black clothes and it's supposedly in there, and it was about to happen soon, i don't know when. >> oh, my god. >> what?

>> this is a real gun. >> reporter: there it is. they finally see the murder weapon. >> actually, it was a good three or four, maybe up to five minutes of him having the gun out and getting other things out after noah had already texted the keyword, and that the cops finally did show up to where we were. >> everybody put your hands out of the vehicle now. >> what the [ bleep ], dude? what the [ bleep ]? >> raise your hands. out of the vehicle. >> reporter: it actually goes down in less than 90 seconds. police spring, guns drawn. >> step out of the vehicle. >> riley, he's just freaking out. he's swearing left and right. he says, "i can't go to jail. i can't go to jail." >> reporter: outside, detectives say their suspect riley appears broken, finally defeated. >> he was taken into custody, he was cooperative and he didn't say anything.

>> we didn't suspect this would be the person we were filming at 11:00 p.m. as the suspect. >> tonight, a maryville college football player behind bars, accused of killing a knox county teenager. >> investigators believe he was trying to destroy evidence. >> reporter: 18-year-old riley gaul in custody, charged with first-degree murder and six other felonies. >> it was a major relief to me, knowing that, you know, he's gonna pay for what he done. >> reporter: and investigators believe he's responsible for even more than emma's murder. inside the white trash bag, not only the murder weapon, but a treasure trove of evidence. what was in there with the gun? gloves? >> there was gloves and trash bags and tennis shoes.

the black tape, some of the black clothing. >> reporter: you think those were the shoes that were used? >> i really think that was the clothes he used, not only the night of the murder, but also that was clothes that was used the morning of the man in black incident. >> reporter: that's right, the last four days all make sense now. authorities convinced that riley gaul is not only a killer but staged his own kidnapping and is also the mysterious man in black who stalked emma. when we come back, the football star on trial with a surprising defense. >> we looked at each other and said -- >> did he just say that? >> reporter: and emma's parents come face to face with their daughter's accused killer. what are your feelings toward him, mark? >> i don't know that i can tell you that on camera.

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>> reporter: nearly two years since emma's death, it's finally judgment day for riley gaul. his appearance radically transformed. >> this was a very different person that i didn't recognize. every day he dressed in a polo, slacks, didn't look up, didn't talk to anyone. >> reporter: more surprising, the defense's first words. >> we'll ask you to find mr. gaul guilty of reckless homicide. >> reporter: the defense lawyer, wesley stone, stunning the courtroom. admitting that his client fired that fatal bullet but not to kill emma, to scare her in hopes that she would call him for help. he would emerge as her hero. >> to me, the hero defense was laughable. >> in his mind, "oh, i'm just gonna scare her by shooting at her through the wall where she's laying her head." if it was his desperate attempt to get attention from her, then it was really, truly a bad idea.

>> reporter: prosecutors point to the trajectory of the bullets. remember, emma was shot at twice from two different angles. >> the trajectory was a crossing pattern that came, where both would have struck her. this is an absolute intent to kill. >> reporter: wesley stone says riley gaul was not the mysterious man in black pounding on emma's door. >> did riley gaul murder emma walker? >> by the definition of murder, no. i do believe it was a reckless homicide. >> reporter: after a week of testimony and more than 30 witnesses. >> guilty. >> reporter: and when you heard guilty? >> it's a good feeling. it doesn't bring her back, but that's the best we could get. he shouldn't get to live his r. >> reporter: the former football star sentenced to life in prison. >> take a life, give a life. it's exactly what he deserves, honestly.

>> reporter: why do you think he killed emma? >> if he couldn't have her -- >> if i can't have you then nobody can have you. and so that's really the only thing i can think of. he claimed he loved her, but it wasn't a healthy love, more of just ownership of her he felt. >> reporter: just this afternoon in court -- >> at his sentencing hearing, gaul breaks his silence. offering a scripted apology. >> i'm sorry i took emma away from you. i wanted to scare her. >> you ended her life. no punishment will ever bring her back, but what helps is knowing you can't do this to anyone else. >> it's not okay to tell someone

who they can talk to, and what they can wear. it's not just bruises, it's emotional and controlling. >> i love you so much. >> reporter: the walker family now finding comfort in unexpected surprises emma left behind. you didn't know she had this on? >> no. >> reporter: very cute. do you look back though these things? >> oh, yes. >> reporter: locking emma in their hearts and memories.