Published: Oct 02, 2012 10:54 AM EDT

BOSTON (AP) - The former University of Alabama-Huntsville professor sentenced to life in prison in a shooting rampage that killed three of her colleagues wants to go on trial in the 1986 death of her brother in Massachusetts.
    
Court documents filed by Amy Bishop's lawyer say she objects to a decision by Norfolk District Attorney Michael Morrissey to decline to prosecute her in the killing of 18-year-old Seth Bishop.
    
Attorney Larry Tipton says Bishop wants to prove at trial that she had a "loving and caring relationship" with her brother and that the shooting was accidental.
    
"She wants to use a trial to help demonstrate that she's innocent. She never intended to kill her brother," Tipton said Tuesday.
    
Morrissey said last week that he decided not to move forward with the murder indictment against Bishop because she has already received a sentence of life in prison without parole in the 2010 Alabama killings.
    
"The penalty we would seek for a first-degree murder conviction is already in place," he said.
    
Morrissey said the indictment would be withdrawn "without prejudice," meaning he could reinstate it if something went wrong with the Alabama sentence, though he said he considered that unlikely.
    
A spokesman for Morrissey did not immediately return a call seeking comment Tuesday.
    
Bishop claims she accidentally shot her brother while trying to unload her father's shotgun in the family's Braintree home. Bishop's mother, who witnessed the shooting, backed up her claim.
    
Authorities initially ruled the shooting accidental, but the investigation was re-opened after Bishop was charged with opening fire during a faculty meeting at the university in Feb. 2010, killing three of her co-workers and wounding three others.

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