BOSTON (CBS/AP) — Lawyers for a woman who as a teenager encouraged her suicidal boyfriend to kill himself in dozens of text messages are urging Massachusetts’ highest court to throw out her involuntary manslaughter conviction.

Daniel Marx is an attorney for 22-year-old Michelle Carter. Marx told the Supreme Judicial Court on Thursday that Carter didn’t force Conrad Roy III to take his life. Marx says it was a “tragic decision that he made.”

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Michelle Carter at her sentencing hearing on August 3, 2017 (WBZ-TV)

Marx also argued the only evidence showing Carter, who was 17 at the time instructed Roy, 18, to get back in the truck filling with carbon monoxide was a text she sent to a friend two months later.

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Conrad Roy. (Photo credit: Janice Roy)

Assistant District Attorney Shosana Stern said Carter knew she had “significant leverage” over Roy. Stern says as Roy became more depressed, Carter became more insistent that he go through with killing himself.

Carter was sentenced to 15 months in jail last year but has remained free while she pursues her appeal.  She was not in court Thursday. It’s expected to take months for the Supreme Judicial Court to issue a ruling.

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(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)