(ABC NEWS)- At Central High School in Knoxville, Tennessee, a Friday night in autumn means a football stadium bathed in lights and two teams battling it out on the gridiron in front of a sea of red and black — everyone decked out in support of the home team, the Bobcats.
“(On Fridays) everybody’s excited at school to see how well we do. It’s what our school revolves around,” said Seth Armstrong, a recent Central High graduate who played for the team. “It’s exciting.”
Out in front of the crowd, the marching band and cheerleaders keep the energy high, and in the fall of 2014, there was a new face on the cheerleading squad – a spirited 14-year-old freshman named Emma Walker.
“Emma really took cheerleading seriously,” said Lauren Hutton, who was a senior on the squad when she met Walker and the two became good friends. “She really loved doing it. It was one of her passions. She loved leading and she loved football games. She loved just being part of crowd appeal.”
Early that fall, Walker’s moves on the sidelines caught the eye of an older student, Number 8 – Central High’s wide receiver and then-junior Riley Gaul.
Raised by his mother and grandparents, Gaul was a top student and loved to play video games. Friends described him as a jokester, not the “classic jock” type.
“He was a little nerdy on the side,” said Zach Greene, one of Walker’s friends. “A little to himself … from the outside looking in, you’d think he was just a normal guy.”
When Walker’s parents first met Gaul, they said their first impressions of him were positive.
“Boy next door,” her mother Jill Walker said. “He came in very polite, very nice in the beginning. He was very likable.”
At first, the relationship seemed “just kind of normal,” said Keegan Lyle, one of Walker’s best friends.
“[Gaul] he didn’t really talk to us, her friends, a lot,” she said. “But I was just like, ‘Oh he’s shy’… it just seemed normal. But then after a while was when we got kind of concerned.”
Friends said they grew concerned because it became apparent to many of them that Gaul didn’t want Walker to hang out with anyone but him.
“He became kind of controlling over her, what she did, her activities,” Keegan said.
“He got more possessive and more clingy towards her, and wouldn’t let her do certain things,” Lauren Hutton added.
Over the next two years, Walker and Gaul seemed to become that other classic high school couple, the kind that was always breaking up and getting back together. Friends described “really dramatic” arguments between the two, often over text message or Snapchat posts.
Jill Walker said Gaul would always comment on what her daughter wore, telling her what she should and shouldn’t wear, to the point where she said she ended up saying something to Emma about it.
Then, Lauren Hutton said, things became “intense” between the couple. She said Gaul started waiting for Walker outside of the supermarket where she worked.
“He would just wait outside for hours,” Hutton said.
Lyle said friends told Walker that they didn’t like the way Gaul treated her, but “she just kind of brushed it off,” Lyle said. “She did her own thing.”
According to Walker’s friends, Gaul became aggressive, sending her Snapchat messages that said, “I hate you I hate everything about you” and “you’re the biggest bitch I’ve ever come in contact with.”
One message in particular alarmed Walker’s mother. “You’re dead to me… I’ll check the obituary… —- you,” Gaul wrote.
“We, on one occasion, saw one [that said] ‘I’ll see your name in the obituary,’” Jill Walker said. ‘He wrote that to her, and we questioned him about it… and he said, ‘I was just angry.’ … and that’s when [I] started to get many more red flags.”
Gaul’s friends became concerned with his well-being. So following Emma’s death, they told detectives a secret Gaul had shared with his friend, Alex McCarty, the day after Gaul’s alleged kidnapping.
“He ended up telling me… that he was so fearful for his life that he had stolen his grandfather’s gun and he showed it to me,” McCarty relayed to “20/20”. “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.”
But Walton, another friend, also told detectives that Gaul had asked him how to get fingerprints off a gun.
“He said he was asking for his roommate,” Walton told “20/20.” “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.'”
Detectives brought Gaul in for questioning on Monday about his whereabouts during the previous 72 hours.
Gaul said he thought he had spent Friday night at his friend Noah Walton’s house.
During his two-hour police interrogation, Gaul didn’t referred to Emma Walker by name, only calling her “the girl.”
“The girl, she texted me,” Gaul told detectives.
“Which girl?” Merritt said.
“The one that passed away,” Gaul said.
“When I first met him (Gaul), I thought he might’ve been a grieving boyfriend,’ Det. James Hurst told “20/20.” “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.”
Gaul told detectives he had been trying to speak with Walker that weekend, but she wouldn’t engage with him.
“But she said if I would help her write her paper, she would talk to me,” Gaul told detectives. “And (Sunday night) I used one of my friends’ phones on campus to call her.”
“Our phone call didn’t go very well,” he said. “She just told me a bunch of cruel stuff, and she blocked his (friend’s) number.”
Afterward, Gaul told detectives, he went over to his grandparents’ house briefly and then drove back to his college. Once there, Gaul said, he broke down and cried for “two, three hours” in his car over breaking up with Walker.
But, as he was describing this in the interrogation room, Merritt said Gaul was “emotionless.”
“His interview was probably one of the most disconnected. It almost seemed rehearsed, deliberate,” Merritt told “20/20.”
At this point, detectives said they knew from McCarty that Gaul had shown his friend his grandfather’s gun. Gaul’s grandfather, who had kept the gun in his car, had reported the 9mm handgun missing prior to Gaul being questioned.
Detectives asked Gaul about the gun, but he told them he didn’t know where it was. He denied showing it to McCarty and also denied asking Walton about removing fingerprints from a gun.
Then detectives asked to see his cellphone and Gaul asked them if he was a suspect. “Should you be?,” the detective responded. But Gaul continued to deny having anything to do with Walker’s death.
When Gaul left the sheriff’s office, McCarty said he started getting text messages from him, asking why he had told the police about the gun. In text messages, Gaul asked his friends not to speak to the police anymore.
“He was on edge,” Walton said.
Thinking Gaul was lying to authorities, Walton and McCarty then collaborated with police to help them lay a trap. On Tuesday night – just one day after Walker’s murder, detectives wired the two teens up with microphones, a transmitter where police could listen in, and a video camera hidden in a key fob, and planned out a sting operation to recover the potential murder weapon.
“Those two young men contacted us and asked if they could help us get what they believed to be the murder weapon back,” Merritt told “20/20.” “Obviously there’s concerns for their safety. We went over all that with them and they were still adamant, very adamant that they wanted to do this. They understood the dangers and the concerns, but they wanted to do it.”
The operation was a success and Gaul was arrested. In addition to the gun, police recovered what they believe to be a treasure trove of evidence, including gloves and black clothing, which authorities say point to Gaul also being the “man dressed in black” who was mysteriously at Walker’s door the Saturday morning before she was killed.
At his trial in May 2018, Gaul’s defense attorney argued in court that he had never meant to kill Walker, but had fired the gun to try to scare her and get her attention.
“He never intended to cause her harm, never intended to cause her death,” Wesley Stone, Gaul’s attorney, told “20/20” exclusively. “Consistent with her reaching out to Riley regarding the events Saturday morning …he was attempting to get her to ask him for help again, sort of to be her protector. It’s been in his heart. It’s been in his mind. It’s been in just everything about him. Every day for the rest of his life, wherever that may be, he will have to live with that reality.”
Stone also said Gaul denies being the mysterious “man dressed in black.”
After five hours of deliberation, jurors found Gaul, now 19, guilty of first-degree murder as well as stalking, theft, reckless endangerment and being in possession of a firearm during a dangerous felony.
In the state of Tennessee, a first-degree murder conviction carries an automatic life sentence.
At his sentencing hearing today, Gaul, who did not speak during his trial, apologized to the Walkers for killing their daughter, but stuck by his defense that it was an accidental shooting.
“I’m sorry I took Emma away from you, that I robbed you of the experience of watching your daughter grow up,” Gaul said in court. “What I can do is tell the truth about that night… I wanted to scare her… I never meant to take Emma’s life. Again I am sorry.”
With justice served for Emma Walker’s death, Jill Walker hopes what happened to her daughter can also serve as a warning to others who may be in tumultuous relationships.
“If your boyfriend or girlfriend is telling (you) you can’t go there, or what to wear, or who to hang out with, or who to talk to. … It’s not OK,” she said. “I think when they become quiet and withdrawn, it’s a big sign too. … It’s not just bruises, it’s emotional and controlling.”
Since her death, Emma Walker’s family has tried to keep her legacy alive. Her mother said she loved animals and wanted to be a NICU nurse, so the family has since gotten a dog park and a NICU patient room at East Tennessee Children’s Hospital named after her.
“It’s all things that are a part of Emma and all mean something,” said Jill Walker. She added that she also hopes people remember her by “being kind to others.”