BOSTON -- The former University of Alabama-Huntsville professor sentenced to life in prison for killing three of her colleagues in a shooting rampage wants to go on trial in the 1986 death of her brother in Massachusetts, her lawyer said Tuesday.

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Morrissey said the indictment would be withdrawn "without prejudice," meaning he could reinstate it if something went wrong with the Alabama sentence, though he said he considered that unlikely.

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.

In court documents filed Monday, Tipton argued that inquest and grand jury proceedings that led to Amy Bishop's indictment in Seth Bishop's death were "one-sided" and tilted in favor of prosecutors. He said prosecutors would be unable to present evidence of a motive or intent by Bishop to kill her brother, while the defense would present evidence that she and her brother had a good relationship and credible evidence that she accidentally shot him.

Tipton said the shotgun has a documented history of malfunctioning and misfiring, evidence he said was discounted or ignored by authorities.

He also argues that Morrissey's plan to file a "nolle prosequi," or declaration that he does not intend to prosecute, is an "abuse of authority."

David Traub, a spokesman for Morrissey, said Bishop's filing of her objection "is legally meaningless."

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