A US woman who sent her boyfriend a barrage of text messages urging him to kill himself when they were both teenagers has been convicted of involuntary manslaughter in a trial that raised questions about whether words can kill.

Key points: Michelle Carter encouraged Conrad Roy to take his own life

Michelle Carter encouraged Conrad Roy to take his own life Both had struggled with depression

Both had struggled with depression She now faces up to 20 years in jail

A judge found Michelle Carter caused the death of Conrad Roy III in Fairhaven, Massachusetts July 2014.

She now faces up to 20 years in prison.

The judge focused his ruling on three words Carter said to the 18-year-old Roy after he climbed out of his truck at the scene of his death and told her he was scared.

"Get back in," Carter told Roy, according to a friend who testified that Carter described the conversation in a text message to her about a month after Roy died.

The judge said those words constituted "wanton and reckless conduct".

He said Carter, then 17, had a duty to call someone for help when she knew Roy was attempting suicide. Yet she did not call the police or Roy's family, he noted.

"She did not issue a simple additional instruction: get out of the truck," the judge said.

Both teens struggled with depression

The case provided a disturbing look at teen depression and suicide.

Carter and Roy met in Florida in 2012 while both were on vacation with their families.

Their relationship consisted mainly of texting and other electronic communications. They only met in person a handful of times.

Both teens struggled with depression. Carter had also been treated for anorexia, and Roy had made earlier suicide attempts.

The sensational trial was closely watched in legal circles and a hot topic on social media, in part because of the insistent tone of text messages Carter sent to Roy.

"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 to Roy the day of his suicide.

"I thought you wanted to do this. The time is right and you're ready … just do it babe," she wrote in another text that day.

In the end, the judge found that it was not the coercive text messages that caused Roy's death. It was Carter's insistence that he get back in the truck.

The judge ruled that Carter could remain free on bail but ordered her not to make any contact with Roy's family or leave the state.

A sentencing hearing was scheduled for August 3.

Carter's lawyer, Joseph Cataldo, argued that Roy was determined to kill himself and nothing Carter did could change that.

He said Carter initially tried to talk Roy out of it and urged him to get professional help, but eventually went along with his plan.

The judge said he did not take into account in his verdict Roy's previous suicide attempts.

Concerns verdict 'violates free speech protections'

Roy's father said the family was pleased with the conviction.

"This has been a very tough time for our family, and we'd like to just process this verdict that we are happy with," Conrad Roy Jr said.

Assistant District Attorney Katie Rayburn said the case dealt with important societal issues, "but in the end, the case was really about one young man and one young woman who were brought together by tragic circumstances".

The American Civil Liberties Union denounced the conviction, saying it "exceeds the limits of our criminal laws and violates free speech protections" guaranteed by the Massachusetts and US constitutions.

Matthew Segal, the ACLU's legal director for Massachusetts, called Roy's suicide tragic but said: "It is not a reason to stretch the boundaries of our criminal laws or abandon the protections of our constitution."

AP/ABC