How football star's friends helped police expose him in ex-girlfriend's death: Part 5

Alex McCarty and Noah Walton saw the gun allegedly used to kill Emma Walker in Riley Gaul's possession.
7:34 | 09/15/18

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Transcript for How football star's friends helped police expose him in ex-girlfriend's death: Part 5
order: Engring student ah Walton and buddy Alex Mccarty are cme friend LE killed his high school sweetheamma walker said he ted throw the gun away, if there was a chance he could get rid of turder weap. Hat you shou. Had to St it. Reporter: The twons work ice to layrap. If should help us get what they believed to be the order weapoback. Huge, hugeiece of inrmation. Reporter: That missinpiece of edence. Authorities suspect the shels found at the C from Riley's grandfather's glock.they wire Alex and noahh camera Hidde key fob, LI this one. So you two offer to lp police iing operation. Maybe justice couldn't have been don ifmade the wng move. The C tip of T key down pointing right at Reportet's now Tuy, the day after em death. The guys invite Riley to come ovo Noah's ho games. I don't know who Emma. I put that on my life. I wouldn't lie to rihere, righnow. I don'know who D. Ielman. I wouy in my life. I had at gun because I was I know you don't believe me, you probabon't either wr.orter: At rst Riley emhether to ust his friends, yet he haplan and begins writingcript for them tthrow police off his scent. Tell them you were on L you were drunk and you were high, uraltered, whatever state you give em wasn't a straighwa He cally want T nullify all our previous stents by sang we were under innd we didnnow what we were tng about. Reporter: I said ok. I didn't do it, but I was like, "Oro, my bad I dn't know I was G to get in trouble." O you're pyihim at thipoint? Eah. But I'm trusting you guys, like, with my life because, I something I didn't do.d, are you guys, are you? Rit now. Like, are you about to do anything? Well, cawe go to T bluffs?need to ged of the gun. Athe bluffs? I'm gonna throw it in the water. They'll never fiver. Reporter: The bluffs is a wodo tenneesee river where teens often hang out to drink and get high. Well, do Y guys want T T Reporter: And just like T E trio jumps in Alex's car. Thing is on. Hey, 've got too Seth's H first. Yeah. On him and we have stepfath house. He said he hid it down in the basement. Me and wait in thca He DI come in. He comes back with this trash bag. A bag rash? I just thw it in there. Is actual audio car. Jo throw it and be done. Reporter: A you anxio, LI, you wa to see it right away? Or du know thayoot pace yourlf what are you thinking? We tried a little bit. Did try tooax it. "Dang, I? Let me S, it's cool. I don't see guns a lot." Reporter: And he doesn't buy N HT. He actually was kind of weird about it. Eporter: B Alex and Noah try to keep things ASU telling Jo singing Ang to the radio.?????? and goinon a run for favorite fast food. But what Riley doesn't K litenant STE Sanders iin an unmarked tailing the boys. He take route. Early on you can hear the audio and then sdese it. What are Y all thinking? It'kind of a,oh, no. What do we dw?"it's a distancing. Repor fortunately, three undeehicles are riding alongside them. Ey community and Alex via group text. With the tension mounting, the GU make their way up to this apartment complex. They parked here and th in theehi'd say, ab ten minut or so, talking. He was pug on white gloves and Wai a everything was very meticu about how he gotrything ou enept waiting an kept wti texting a that poi, the police? The bag. I to tidn't see yet, but he had a bag full O black clothes and it's supposedly in there, and about hn soon, I T know when. Oh, my god. WHA This is Al gun. Reporter: There it is they finally see the murder weapon. Ctually, was a good three or four, maybe up to five Tes of himaving the already ted here we Everybody put hands out the vehicle now. What the bleep ], de? What the ? Aise your hands. Out of the vle. Reporte it actual goes down in less than 90 seconds. Po spring, gs Step out of the vehicle. Ril he's sg left and right. Heay "I can't O jail. I cat go to jail." Reporter:utside, decteir suspect rilepears broken, finally defeated. He was tan into cdyhe waoperative D he didn't say anything. E dt suspects would beerson we were filming at 11:00 P.M. As the suspect. Ton football par behind bs, accu Investigators believe he was trying to destroy evidence. Reporter: 18-year-old Riley Gaul in custodrg with first-degree murder and six other fees. It was jor relief T knowing that, you know, he's gonna pay for what he done. Reportend investigators believe he's responsible for even more than Emma's murder. Inside thite trashag, not only the weapon, but a treasure trove of evidence. What W in there with the gun? Gles? There was gloves and tras bags and tennis S. The black tape, some of the blacothing. Orter: Youhihose were the ses that were used? I really think that was clothes he used, not only th night of the murder, but also was clothhat was used the man in blk indent. Reporter: That's right, the last four days all make sense now. Auities convced that riy Gaul is T only a kler but stagis own kidpp and is also the mysterious man lack who slk when wcohe football

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

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