A US court has rejected a plea to dismiss the case against a Massachusetts woman accused of involuntary manslaughter after she sent dozens of texts to her depressed boyfriend urging him to kill himself.

Michelle Carter allegedly sent her vulnerable 18-year-old boyfriend Conrad Roy messages, like "Hang yourself, jump off a building, stab yourself I don't know there's a lot of ways". Roy was found dead in his pickup truck in a K-Mart car park on 12 July 2014.

But defence lawyers said the 20-year-old's actions did not rise to the level of criminality. However, they admitted that it was "reckless", BBC reported.

Joseph Cataldo, Carter's lawyer, requested Bristol Juvenile Court Judge Lawrence Moniz to dismiss the case, saying it lacks "causation", but the judge disagreed.

Prosecutor Katie Rayburn argued that there was no lack of causation as Carter had influenced Roy to "get back in the car", as it was filling with poisonous carbon monoxide gas, during their 46-minute phone conversation on the night of the incident.

Rayburn also claimed that Roy took almost 20 minutes to die, giving an ample amount of time to Carter, who was 17-year-old at that time, to call someone for help but she did not.

She added that Carter took advantage of Roy's vulnerability and compelled him to kill himself in order to get attention as "the grieving girlfriend".

Rayburn rested the case on Thursday, saying Carter's texts amounted to: "I love you. Kill yourself."

But on Friday, 9 June, Cataldo added that it could not be proven that she caused her boyfriend's death as Massachusetts is among 10 US states that do not criminalise assisted suicide.

Michelle Carter
Michelle Carter allegedly sent her vulnerable 18-year-old boyfriend Conrad Roy messages, like "Hang yourself, jump off a building, stab yourself I don't know there's a lot of ways” - File photo Facebook

The defence lawyers also said that Carter's texts could not be taken as evidence against her as her words fall under free speech, protected by the first amendment of the US constitution.

The plea to dismiss the case from the defence side came a day after some of Carter's friends testified about their conversations with her in the days after Roy's death.

One of her friends said that after Roy died, Carter said her message saying, "Sam [the victim's] death is my fault like honestly I could have stopped him I was on the phone with him and he got out of the [truck] because it was working and he got scared and I f****** told him to get back in Sam because I knew he would do it all over again the next day and I couldn't have him live the way he was living anymore I couldn't do it I wouldnt let him."

Alexandra Eitheir, another friend of the accused, said Carter wrote her that she was "was on the phone talking to him when he killed himself. I heard him dying".

Carter had also contacted a friend when she got know that investigators were looking through Roy's phone. "They read my text messages to him I'm done", the accused wrote, adding, "his family will hate me and I could go to jail", according to court hearings.