A young Massachusetts woman who sent her boyfriend dozens of text messages telling him to kill himself will stand trial for involuntary manslaughter, a court has ruled.

Michelle Carter, then 17, allegedly sent messages to her boyfriend Conrad Roy III, 18, encouraging him to follow through on his plans to take his own life, including telling him to “get back in” a truck filled with carbon monoxide fumes.

In a series of texts and online messages, Ms Carter convinced Mr Roy that his death would be painless, and his parents were prepared for him to commit suicide.

She frequently encouraged him to kill himself in the short term: “when are you going to do it?”, “if you don’t do it tonight, you’re going to be miserable”, “you just have to do it… tonight is the night, it’s now or never”.

The court heard that, when Mr Roy expressed a plan to kill himself on a Saturday night, she replied “today is only Monday”.

The Supreme Judicial Court ruled on Friday that a grand jury had probable cause to indict Michelle Carter in Mr Roy’s death, after hearing evidence suggesting she engaged in a “systematic campaign of coercion”, and her instruction to “get back in” the truck in order to poison himself was a “direct, causal link” to the teenager’s death.

“In sum, we conclude that there was probable cause to show that the coercive quality of the defendant’s verbal conduct overwhelmed whatever willpower the 18-year-old victim had to cope with his depression, and that but for the defendant’s admonishments, pressure and instructions, the victim would not have gotten back into the struck and poisoned himself to death,” Justice Robert Cordy wrote for the court in the unanimous ruling.

After sending Mr Roy instructions on how to give himself carbon monoxide poisoning, and carefully planning the act with him so as to make sure the attempt did not fail, Ms Carter sent him a message after 4am on 12 July 2014 saying: “You said you were gonna do it. Like I don’t get why you aren’t.”

“Last night was it,” she told him. “You kept pushing it off and you say you’ll do it, but you never do. It’s always gonna be that way if you don’t take action.”

Later that day, when Mr Roy expressed doubts, telling Ms Carter he felt hesitant, she reassured him he would be “free and happy” if he committed suicide, and he should not put it off any longer.

“It can’t even wait till tonight,” she told him.

That evening, Mr Roy died of carbon monoxide poisoning in a Kmart car park. Phone records show he had called Ms Carter just after 7pm, and their phones had been connected for 47 minutes.

Phone records reveal that at some point Mr Roy got out of the car, and said he was scared because the carbon monoxide poisoning was working. Carter, the court heard, told him to “get back in”.

Following Mr Roy’s death, Ms Carter sent a series of messages implying she did not know what had happened to her boyfriend, including a message to his sister asking “do you know where your brother is?”

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Messages Ms Carter sent to a friend reveal she pretended to have had no knowledge of what Mr Roy planned to do, saying she felt stupid because she “should have knew”.

Prior to Mr Roy’s death, Ms Carter had texted the same friend saying she was worried because she had not heard from him, but at the same time was messaging Mr Roy about how to fix the generator he had stolen from his father in order to kill himself.

Carter’s lawyer, Joseph Cataldo argued her texts were protected by the First Amendment – her right to free speech – and she had not caused Mr Roy to kill himself.

He argued Mr Roy was depressed, and had previously attempted to take his own life. It was also argued that Ms Carter could not be charged with manslaughter, because there is no law in Massachusetts against encouraging or assisting suicide.