A teenager on trial in the US for allegedly coaxing her boyfriend in a series of text messages to kill himself has been accused of helping him to research the "best way" to do so, according to court documents.

Michelle Carter, 18, is defending a charge of involuntary manslaughter following the death of her boyfriend, Conrad Roy III, also 18, who was found dead in his car in a parking lot in Massachusetts in July last year.

Prosecutors in the New Bedford Juvenile Court claim that Ms Carter, who was 17 at the time, discussed with Mr Roy the best method to make his death "painless and quick".

Among the hundreds of text messages the pair allegedly shared, Ms Carter is accused of going so far as to provide "back-up plans" should his initial choice not work.

An indictment issued by a Bristol County grand jury alleges that, between July 6 and July 12, Ms Carter assisted Mr Roy to take his life via dozens of messages.

She allegedly counselled him to overcome his doubts about suicide, and allegedly texted him comments such as "tonight is the night" and "you just have to do it", according to court documents.

In one text she allegedly said to Mr Roy: "You said you were gonna do it. I don't get like why you aren't."

Mr Roy responded: "I don't get it either. I don't know."

In court last week, Ms Carter's attorney, Joseph Cataldo, attempted to get the charge against his client dropped, claiming the Mr Roy had "brainwashed" Ms Carter, the Boston Herald reported.

Mr Cataldo argued that Ms Carter at first tried to discourage Mr Roy from killing himself and even suggested he join her at McLean Hospital, a psychiatric hospital where she had been receiving treatment.

Mr Roy had tried to get Ms Carter to take part in a Romeo and Juliet-style suicide the month before his death, Mr Cataldo said.

The lawyer said that Mr Roy told Ms Carter: "Let's do a Romeo and Juliet. The two of us, together, kill ourselves."

Mr Cataldo said that Ms Carter replied: "[Expletive] no. We are not dying."

Eleven days before his death, Mr Roy again asked Ms Carter to kill herself with him, Mr Cataldo said, and she said no.

Eventually, Mr Cataldo said, "There's a turn.

"He has, in fact, brainwashed her to the point where she's now accepting his idea of, 'This is my only option,' " Mr Cataldo said.

Ms Carter, who was on the phone to Mr Roy when he died, later told a friend she was talking to him while he killed himself and heard him cry in pain.

Prosecutors allege Ms Carter text messaged Mr Roy's sister on the night of his death and acted as though she did not know what had happened.

"Do you know where your brother is?" she allegedly asked Mr Roy's sister in a text.

A few days after Mr Roy's death, Ms Carter told a friend via text message that she blamed herself.

"It's my fault, I was talking to him while he killed himself. I heard him cry in pain. I should have known. I should have done something."

The case continues.

Fairfax Media

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