WASHINGTON — The Supreme Court on Monday left in place the conviction of a Massachusetts woman who sent her boyfriend text messages urging him to kill himself.

Michelle Carter is serving a 15-month sentence after being convicted of involuntary manslaughter in the 2014 death of her boyfriend, Conrad Roy III. A judge determined that Carter, who was 17, caused the death of the 18-year-old Roy when she ordered him in a phone call to get back in his carbon monoxide-filled truck that he’d parked in a Kmart parking lot.

The phone call wasn’t recorded, but the judge relied on a text Carter sent her friend in which she said she told Roy to get back in. In text messages sent in the days leading up to Roy’s death, Carter also encouraged Roy to follow through with his suicide plan and chastised him when he didn’t, Massachusetts courts found.

The case garnered national attention and sparked legislative proposals in Massachusetts to criminalize suicide coercion.

Carter’s lawyers argued in their Supreme Court appeal that the conviction should be thrown out because it was an “unprecedented” violation of her free speech rights that raised crucial questions about whether “words alone” are enough to hold someone responsible for other person’s suicide.

The lawyers also argued there was simply not enough evidence to prove Carter urged Roy to get back in his truck to die, or that he would have lived if she had called for help or taken other actions to try and save his life.

Joseph Cataldo, one of Carter’s lawyers, said Monday’s decision was an “injustice” and that the legal team is weighing its next steps. He didn’t elaborate.

“The U.S. Supreme Court not accepting Michelle Carter’s petition at this time is unfortunate,” he said in a statement. “Clearly many legal scholars and many in the legal community understand the dangers this precedent created by the Massachusetts courts.”

Carter has been serving her sentence at the Bristol County House of Correction in Dartmouth, Mass.

She was initially set to be released in May, but is now scheduled for release Jan. 23. Jail officials said Carter has accrued enough “good time” credits for good behavior and attending programs while incarcerated after she was denied parole last September.