Michelle Carter jailed for 15 months for encouraging teen boyfriend to suicide

The father of a teenager who killed himself after his girlfriend encouraged him in dozens of text messages, has cried during her sentencing, accusing the young woman of "exploiting" his son's weaknesses.

Michelle Carter, who encouraged her suicidal boyfriend to kill himself in dozens of text messages and told him to "get back in" a truck filled with toxic gas, has been sentenced to 15 months in jail for involuntary manslaughter.

The Massachusetts woman was convicted in June by a judge who said her final instruction to Conrad Roy III caused his death.

Carter was 17 when the 18-year-old Roy was found dead of carbon monoxide poisoning in July 2014.

Juvenile Court judge Lawrence Moniz on Thursday gave Carter a two-and-a-half-year jail sentence but said she had to serve only 15 months of that. He also sentenced her to five years of probation.

The victim's father Conrad Roy Jr wept while reading his statement in court.

"We all felt he was heading in the right direction and over the worst of it," he said of his son's depression.

"He had such a bright future and Michelle Carter exploited my son's weaknesses.

"She used him as a pawn and she has not shown any remorse. Where was her humanity? How he could she behave so viciously?"

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Carter's lawyer, Joseph Cataldo, asked the judge to spare his client any jail time and instead give her five years of probation and require her to receive mental health counselling.

He said Carter was struggling with mental health issues of her own - bulimia, anorexia and depression - during the time she urged Roy to kill himself.

Prosecutor Maryclare Flynn sought the maximum sentence of 20 years behind bars.

In dozens of text messages, Carter had urged Roy to follow through on his talk of taking his own life. "The time is right and you are ready... just do it babe," Carter wrote in a text the day he killed himself.

The sensational trial was closely watched on social media, in part because of the insistent tone of Carter's text messages.

"You can't think about it. You just have to do it. You said you were gonna do it. Like I don't get why you aren't," Carter wrote in one text.

Assistant District Attorney Maryclare Flynn told the court "Michelle Carter - her actions - killed Conrad Roy. She ended his life to better her own. She has been convicted of a very serious crime that merits serious punishment."

In convicting Carter, the judge focused his ruling on Carter telling Roy to "get back in" after he climbed out of his truck as it was filling with carbon monoxide and told her he was afraid.

The judge said those words constituted "wanton and reckless conduct" under the manslaughter statute.

Carter and Roy met in Florida in 2012 while both were on vacation with their families. After that, they only met in person a handful of times. Their relationship consisted mainly of texting.

If you are concerned about the mental health of yourself or a loved one, seek support and information by calling Lifeline 13 11 14, the Suicide Call Back Service 1300 659 467, or Beyond Blue 1300 22 46 36.