DURHAM (WTVD) -- A key witness for the prosecution spent hours fielding intense questions and criticism from the defense in the final day of testimony in the Joseph Mitchell murder trial.
"You understand it's important to be accurate as it is to be truthful," said defense attorney Jay Ferguson during a grueling cross examination.
A forensic psychologist testifying on Monday told a jury that the Durham father accused of killing his youngest son, Blake, in September 2010 and attacking his oldest son and daughter who were 10 and 13 at the time -- is a narcissist not a sleepwalker.
"I diagnosed him with narcissistic personality disorder," said Dr. Nancy Laney, emphasizing that during her four interviews with Mitchell he seemed to have a lack of empathy and a tendency to inflate his abilities and accomplishments.
She added, "There's no evidence to suggest Mr. Mitchell was suffering from any type of mental disease or defect or condition that rendered him unconscious at the time of the alleged offense."
The defense questioned whether Laney displayed confirmation bias, an expert's tendency to build a case around the investigator's theory.
Ferguson questioned her testimony that she had spent 14 hours interviewing Mitchell. She would admit on the stand Tuesday morning that she overestimated the 4 hours she observed the defendant.
The defense tried to further discredit her testimony by highlighting her claims Mitchell never mentioned the impact of his son's murder on his wife and surviving children.
Ferguson reminded the jury Mitchell wrote letters to his wife and children. The expert witness said it was information Mitchell didn't provide during his interview with her.
As for the state's claims Mitchell had not had a sleepwalking incident since the murder, the defense told the jury he was medicated while in jail which allowed him to sleep.
Closing arguments began Tuesday afternoon.
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Closing arguments begin in 'sleepwalking' Durham murder trial