BOSTON » Time could be running out for Massachusetts prosecutors seeking to put a teenager on trial for encouraging her boyfriend to take his own life by sending him dozens of text messages and telling him to “get back in” a truck filled with carbon monoxide fumes.

Michelle Carter, 19, is charged with involuntary manslaughter in the 2014 death of Conrad Roy III, 18, of Mattapoisett.

Because she was charged as a youthful offender, she could face up to 20 years in prison, the same sentence an adult would if convicted. But Carter’s lawyer has asked the state’s highest court to either dismiss the manslaughter charge or require prosecutors to try her as a juvenile.

If the Supreme Judicial Court agrees to send the case to juvenile court, prosecutors will have only until Aug. 11 to put her on trial. That’s the day Carter turns 20, when she will age out of the juvenile system.

But if the court upholds the prosecution’s decision to charge her as a youthful offender, there is no race against the clock for putting her on trial and she could face adult punishment.

For Roy’s family, the possibility that Carter could receive no punishment is difficult to fathom.

“We’re anxiously awaiting the decision,” said Janice Roy, Conrad’s grandmother. “We’re hoping that she is put on trial.”

The charge against Carter, of Plainville, drew national attention after transcripts of text messages were released publicly, showing she urged Roy to follow through on his plan to take his own life.

“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, then 17, wrote to Roy the day of his death.

The teens had met in Florida two years earlier while visiting relatives. Their relationship consisted mostly of text messages and emails, and they hadn’t seen each other in more than a year when Roy died, even though they lived only about 50 miles apart in Massachusetts.

When Roy’s body was discovered in his pickup truck in Fairhaven, police found a gasoline-operated water pump in the back seat.

Carter’s lawyer, Joseph Cataldo, argues that her text messages were free speech protected by the First Amendment and do not constitute a crime. Massachusetts does not have a law against encouraging or assisting suicide.

Cataldo said Carter had repeatedly tried to talk Roy out of taking his own life, but gave up about two weeks before his death. He argued that Roy — who had made a previous suicide attempt — was determined to end his life.

“She did not cause his death. He obviously caused his own death and took all the necessary steps — 100 percent of the physical steps — to bring about his own death,” Cataldo said.

But during arguments before the Supreme Judicial Court in April, Assistant District Attorney Shoshana Stern said Carter engaged in “emotional manipulation” of a vulnerable teen who had struggled with depression. Stern said that in addition to sending him text messages, Carter also spoke on the phone with Roy while he was in his truck inhaling carbon monoxide fumes. At one point, he got out of his truck, but Carter told him to “get back in,” Stern said.

Peter Elikann, a Boston defense attorney who is not involved in the case, said that if the court allows Carter to be tried as a youthful offender, prosecutors may still have a tough time winning a manslaughter conviction.

“The issue will be whether she forced him to get back into the truck, which would mean she did, in fact, participate,” Elikann said. “Or did she just encourage him, which would be perhaps a reprehensible act, but not illegal.”

A juvenile court judge denied a defense motion to dismiss the manslaughter charge, citing records prosecutors say show Carter was on the phone with Roy for about 45 minutes while he was inhaling carbon monoxide, heard him moaning, did not call 911 and told him to get back in his truck when he became afraid.