The trial against former Maryville College football player William Riley Gaul began Tuesday in Knoxville, Tennessee.
The 19-year-old stands accused of murder for killing his ex-girlfriend Emma Walker a little more than a year-and-a-half ago.
Gaul killed the 16-year-old while she was sleeping in her bed in the early hours of November 21, 2016, by firing two shots through her bedroom wall from outside the house. Her mother found her dead the next morning.
While both sides agree that Gaul killed Walker, his lawyers say the shooting was an accident - part of a plot to get her back.
The trial against former Maryville College football player William Riley Gaul (pictured above in court on Wednesday) began Tuesday in Knoxville, Tennessee
The 19-year-old (left) stands accused of murder for killing his ex-girlfriend Emma Walker (right) a little more than a year-and-a-half ago
Gaul's lawyers said he shot at the bedroom to try and frighten Walker, so that she would call him for help and he could run to her 'rescue' and be her 'hero'. They claim he didn't know bullets could pierce through walls.
They say he can't be found guilty of first-degree murder since he never intended to kill Walker.
'Intentionally doing an act and intending to kill are two entirely different things,' defense attorney Wesley Stone said in his opening statement on Tuesday. 'I think throughout this trial, that distinction will become apparent.'
But Knox County Assistant District Attorney Kevin Allen called Gaul a 'premeditated cold-hearted killer'. He said Gaul fatally shot Walker, aiming directly at where she was sleeping in bed, in anger over the end of their tumultuous two-year relationship.
Gaul killed Walker by firing two shots through her bedroom window. Her mother Jill, pictured above on Tuesday, discovered her dead the next morning
Walker's mother Jill was among the first to take the stand on Tuesday, on the first day of the trial.
She spoke about how her daughter starting dating Gaul when she was a freshman and he was a junior at Central High School.
'In the first couple months it was fine,' Jill Walker testified this week. 'It seemed normal. We kept a close eye, she was only 14 years old at the time.'
But Jill and her husband Mark monitored their daughter's messaging apps and didn't like the way that Gaul was speaking to her.
Gaul, a former Maryville College football player, faces the prospect of life in prison without parole if he is found guilty of first-degree murder
'Some of the language and the way she was spoken to, we didn't think it was appropriate — to speak to anyone like that,' she said. 'We tried to discourage her from seeing him. We would set limits how and when they could see each other.'
When Gaul left for college at the beginning of the 2016-2017 school year, they hoped the two would break up. But their daughter was 'stubburn' and the relationship continued off-and-on for a period. Finally, it was Walker who decided to end things not long before her death.
'She had chosen to move on,' Jenny Weldon, Emma's aunt, told the court. 'He refused to accept it. He chose not to accept her wishes.'
On Tuesday, five of Walker's friends testified about a strange interaction they had with Gaul the Friday before Walker's death.
The group of friends was having a sleepover after a Friday football game when Walker started getting strange text messages from an unknown number.
'Go to your car with your keys,' the first text read. 'Go alone … I've got someone you love. If you don't comply I will hurt them.'
Walker showed the text to friend Zach Greene, and they walked outside together but saw nothing.
Then another text message came in.
'What part of alone wasn't clear? I don't want to hurt a loved one.'
Zach Greene was with Walker two days before her death, when Gaul tried to fake a kidnapping to win her back
Gaul's grandfather, James Walker (no relation to Emma),