Washington (CNN) The Supreme Court said on Monday that it would not take up the case of Michelle Carter, a woman serving a 15-month sentence for her part in her boyfriend's death by suicide.

Carter had asked the justices to take up her case, arguing that her conviction three years ago for involuntary manslaughter -- based on her words alone -- violated her First Amendment rights.

Carter was found guilty in 2017 of involuntary manslaughter in the death of 18-year-old Conrad Roy III, who killed himself in his car in Fairhaven, Massachusetts, in 2014. After his death, investigators discovered Roy had texted Carter as he contemplated and attempted suicide, and that she'd encouraged him to do it when he had doubts.

In one of the dozens of texts Carter sent to Roy before his death, she wrote: "I thought you wanted to do this. The time is right and you're ready, you just need to do it! You can't keep living this way."

Massachusetts' highest court upheld Carter's conviction following an appeal last February.

Read More