TAUNTON, Mass. — For a case that had played out in thousands of text messages, what made Michelle Carter’s behavior a crime, a judge concluded, came in a single phone call. Just as her friend Conrad Roy III stepped out of the truck he had filled with lethal fumes, Ms. Carter told him over the phone to get back in the cab and then listened to him die without trying to help him.

That command, and Ms. Carter’s failure to help, said Judge Lawrence Moniz of Bristol County Juvenile Court, made her guilty of involuntary manslaughter in a case that had consumed New England, left two families destroyed and raised questions about the scope of legal responsibility. Ms. Carter, now 20, is to be sentenced Aug. 3 and faces up to 20 years in prison.

The judge’s decision, handed down on Friday, stunned many legal experts with its conclusion that words alone could cause a suicide.

“This is saying that what she did is killing him, that her words literally killed him, that the murder weapon here was her words,” said Matthew Segal, a lawyer with the American Civil Liberties Union of Massachusetts, which raised concerns about the case to the state’s highest court. “That is a drastic expansion of criminal law in Massachusetts.”