DOVER, N.H. — The defense attorney for a man charged with raping and killing a University of New Hampshire student spent his first day cross-examining the state’s star witness trying to convince jurors she was possessed by imaginary characters she had created.

Defense attorney Joachim Barth also asserted the witness, Kathryn McDonough, changed her story and shifted the blame from herself to defendant Seth Mazzaglia, her ex-boyfriend, just to get a reduced sentence.

Mazzaglia is charged with first-degree murder in the October 2012 death of 19-year-old Elizabeth “Lizzi” Marriott, of Westborough, Mass. He has pleaded not guilty. Prosecutors say he and McDonough dumped Marriott’s body in a river and it hasn’t been recovered.

McDonough, 20, testified Thursday the imaginary characters she began creating during childhood were to help her get through difficult situations.

Barth opened his cross-examination reminding McDonough that she gave a statement to him and his investigators two days after Mazzaglia’s arrest and told them Marriott died while McDonough was having sex with her with a restraint that kept Marriott from raising her head. McDonough told the defense team that Marriott had a seizure and died.

McDonough agreed that was what she said at the time.

Barth emphasized that she changed her story only after she was given an immunity deal “to reduce your years in prison.” McDonough agreed with the timing.

Barth spent several hours going over Facebook pages dating to 2011 and other conversations with Mazzaglia, 31, and others in which she talked of other characters or personas she had created. She testified repeatedly Thursday that they were “masks” she would use when she wanted to be stronger.

During questioning peppered by objections by prosecutors, Barth referenced conversation after conversation in which McDonough had talked about the personas, including a strong one dubbed Scarlet.

Barth told jurors during his opening statement May 28 that McDonough had “a real problem. She had five voices in her head.” And he told them McDonough was secretly recorded telling a friend that the night Marriott died one of the personas took control and McDonough blacked out.

On cross-examination, McDonough disputed that her reference was to the night Marriott died. She said that she told Roberta Gerkin that the persona was taking over during that conversation and blocking her memory.

“I was telling her Anay, or Charlotte, was blocking my memory of that night so I wouldn’t have to tell her about it,” McDonough said.

She denied that Charlotte, who Mazzaglia referred to as Annay, interfered with her thinking or caused her to black out.

“She’s just a character I came up with, not someone who can take over my mind or control me,” McDonough said.

Barth said she repeatedly referred to her characters in texts and Facebook conversations with Mazzaglia and battled her assertions that she didn’t take them seriously. She said she invoked them because Mazzaglia took his own personas very seriously.

“I was just a 17-year-old kid,” she testified Thursday. “I had just met his person. When I mentioned it (a persona) he took it very seriously.”

McDonough testified Wednesday she lured Marriott, her co-worker, to the apartment with the promise of watching a movie or playing a video game. In reality, she testified, she was offering Marriott as a sex partner for Mazzaglia, who was angry that McDonough had left him alone for 12 days.

But McDonough said Marriott twice refused sexual suggestions by Mazzaglia.

“That’s when he moved up behind her and he pulled the rope up over her head and he strangled her,” she said.

She said Mazzaglia raped Marriott while calling her names as her motionless body lay on the floor.

Cross-examination resumes Friday.

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