The court heard that Carter and Roy exchanged more than 20,000 text messages - with more than 1,000 of those messages sent in the days leading up to Roy's death. Prosecutors say Carter sent her boyfriend dozens of text messages urging him to take his own life.
Carter's lawyer argue that she tried repeatedly to talk him out of it and only began to support the plan when it became clear he would not change his mind.
They add that the texts are are protected under free speech and that Roy was depressed and previously tried to take his own life.
Carter was 17 when Roy took his own life via carbon monoxide poisoning and prosecutors argue she is criminally responsible because she encouraged him to kill himself in a series of text messages.
In the hour before his death, prosecutors say, he was on the phone with Carter for 47 minutes, at one point telling her he was getting out of the truck because he feared it wasn't working.
'The car was filling up, and he was scared,' said prosecutor MaryClare Flynn. 'She told him to get back in the car.'
'When he actually started to carry out the act, he got scared again and exited his truck, but instead of telling him to stay out of the truck ... Carter told him to 'get back in',' the police report said.
Days before the suicide, prosecutors say Carter wrote, 'Don't be scared. You're finally to be happy in Heaven.'
'When are you going to do it? Stop ignoring the question???? You can't keep push [sic] it off,' prosecutors claimed Carter texted Roy.
Roy had also attempted suicide and had been hospitalized before he met Carter.
In a text message sent two weeks before his suicide, he wrote of his desire to end his own life. 'I can't get better,' he wrote. 'I already made my decision.'
Roy's body was found in the truck in Fairhaven, Massachusetts on July 13, 2014, after his parents reported him missing, and police found the text messages after looking through his phone.
Police found a gasoline-operate water pump in the back seat of the truck after he died. A coroner later ruled he died of carbon monoxide poisoning.
Carter messaged one of her friends nearly a week later expressing her fears that her messages to Roy had been found, People reported.
'I just got off the phone with Conrads mom about 20 mins ago and she told me that detectives had to come and go thru his things and stuff,' she wrote.
'They read my messages with him I'm done... His family will hate me and I could go to jail.'
The case involving Carter has drawn national attention after transcripts of those text messages were made public.
Even though Carter, who met Roy while on a vacation to Florida several years earlier, reportedly called herself his 'girlfriend', his friends and family said they never knew of the relationship.
Carter had also repeatedly posted online about how much she missed him, saying at one point, 'I will never understand why this had to happen.'
In March, a controversial psychiatrist said Carter was 'involuntarily intoxicated' by prescribed antidepressants when she texted her boyfriend encouragement as he committed suicide.
Psychiatrist Dr Peter Breggin testified that Carter was not responsible because she was being treated with the antidepressant Celexa in 2014, which would have altered her brain and meant she wouldn't have understood the impact of her texts to Roy.
'She had no notion of wrongfulness on what she was doing,' Dr Breggin told the court, according to 12WPRI.
Dr Breggin, who was hired by defense attorneys, said Carter loved Roy but experienced a metamorphosis due to her medication.
'The impairment of being on the drugs while you are growing up - while you're brain is maturing, while your frontal lobes are developing - you're talking about being inundated with neurotoxic effects,' he said.
Prosecutors criticized Dr Breggin's credentials during cross-examination and sought to discredit him as an 'extremist' who was critical of antidepressants.
Assistant District Attorney Katie Rayburn pushed Dr Breggin to admit he only reached his conclusion about Carter based on medical records and the text exchange between her and Roy.
The judge did not rule on whether Dr Breggin can testify but the psychiatrist has been previously banned from testifying in other cases.
Carter's defense team were granted a request late last year for funds to hire an expert to examine the antidepressant she was taking at the time.
Her attorney Joseph Cataldo said at the time the drug Celexa could be the key to her defense because it can increase suicidal thoughts.
For confidential help, call the National Suicide Prevention Lifeline at 1-800-273-8255 or visit suicidepreventionlifeline.org.
'IT'S NOW OR NEVER': MICHELLE CARTER'S TEXT MESSAGES TO CONRAD ROY
Prosecutors say Michelle Carter sent her boyfriend, Conrad Roy III, dozens of text messages urging him to take his own life.
Carter's lawyer argue that she tried repeatedly to talk him out of it and only began to support the plan when it became clear he would not change his mind.
Here are excerpts from their text exchanges, with messages cited by her lawyer first, followed by those cited by prosecutors:
June 29, 2014:
Carter: 'But the mental hospital would help you. I know you don't think it would but I'm telling you, if you give them a chance, they can save your life'
Carter: 'Part of me wants you to try something and fail just so you can get help'
Roy: 'It doesn't help. trust me'
Carter: 'So what are you gonna do then? Keep being all talk and no action and everyday go thru saying how badly you wanna kill yourself? Or are you gonna try to get better?'
Roy: 'I can't get better I already made my decision.'
July 7, 2014:
Roy: 'If you were in my position. honestly what would you do'
Carter: 'I would get help. That's just me tho. When I have a serious problem like that, my first instinct is to get help because I know I can't do it on my own'
Roy: 'Well it's too late I already gave up.'
Between July 6, 2014 and July 12, 2014:
Carter: 'Always smile, and yeah, you have to just do it. You have everything you need. There is no way you can fail. Tonight is the night. It's now or never.'
Carter: '(D)on't be scared. You already made this decision and if you don't do it tonight you're gonna be thinking about it all the time and stuff all the rest of your life and be miserable.
You're finally going to be happy in heaven. No more pain. No more bad thoughts and worries. You'll be free.'
Carter: 'I just want to make sure you're being serious. Like I know you are, but I don't know. You always say you're gonna do it, but you never do. I just want to make sure tonight is the real thing.'
Carter: 'When are you gonna do it? Stop ignoring the question'
Carter: 'You can't keep living this way. You just need to do it like you did the last time and not think about it and just do it, babe. You can't keep doing this every day.
Roy: 'I do want to but I'm like freaking for my family I guess. I don't know.'
Carter: 'Conrad, I told you I'll take care of them. Everyone will take care of them to make sure they won't be alone and people will help them get through it. We talked about this and they will be okay and accept it. People who commit suicide don't think this much. They just could do it.'