(New throughout, updates with comments from Bulger attorney and niece of alleged victim)
By Scott Malone
BOSTON, Feb 13 (Reuters) - Prosecutors urged a federal judge on Wednesday to reject accused Boston mob boss James "Whitey" Bulger's claim that he cannot be tried for 19 killings because former prosecutors gave him immunity.
Bulger, who fled Boston in 1995 after receiving a tip from a corrupt FBI agent that arrest was imminent, has argued that a former top federal prosecutor, now deceased, promised not to prosecute him for crimes committed by the "Winter Hill" gang he is accused of leading. Bulger says the immunity was granted because he provided information on rival crime organizations.
At a pre-trial hearing on Wednesday, prosecutors challenged that argument, saying it would mean former Assistant U.S. Attorney Jeremiah O'Sullivan, now deceased, essentially would have been authorizing Bulger to commit murder.
"Mr. O'Sullivan does not have that authority. As a matter of law, an assistant U.S. attorney does not have authority to allow the killing of U.S. citizens," said Zachary Hafer, one of the assistant U.S. attorneys prosecuting the case. "Any contract between Mr. O'Sullivan and Mr. Bulger, as it contemplated murder, would be void."
Bulger's attorney, J.W. Carney of the Boston law firm Carney & Bassil, countered that an immunity claim was possible, but that its validity should be determined at Bulger's trial, scheduled to begin in June.
"James Bulger will testify that he was given immunity from prosecution by Jeremiah O'Sullivan," Carney said at U.S. District Court in Boston.
Carney provided no details on Bulger's agreement with O'Sullivan, saying that evidence would be best presented at trial.
BULGER 'NEVER AN INFORMANT'
After the hearing, Carney told reporters that Bulger had never been an informant, though he declined to say why prosecutors would have given his client immunity if he had not exchanged information.
"James Bulger was never an informant to the FBI or anybody else at any time," Carney said. "It is not the reason he received immunity."
Law enforcement officials have repeatedly described Bulger as part of a network of informants in the Boston mob.
Carney told reporters he would not disclose the reason for Bulger's immunity before trial.
Bulger, now 83, spent 16 years in hiding after fleeing the city. His name was prominent on the FBI's "Ten Most Wanted" list until his arrest in California in June 2011.
He has pleaded not guilty to all charges. If convicted, Bulger, who was not present in court, faces a maximum sentence of life in prison.
Family members of some of his victims were present at court on Wednesday, and reacted with horror to the idea of Bulger claiming immunity.
"This fact of Bulger saying he had immunity is sick. That does not allow you to commit murder," said Sandra Patient, of Manchester, New Hampshire, whose uncle, Arthur "Bucky" Barrett was among Bulger's alleged victims. "There is no way this guy is going to walk on 19 murders."
Prosecutors argued in court papers that Bulger's decision to go into hiding undercuts his claim that he had a deal with O'Sullivan, who died in 2009, protected him from prosecution.
O'Sullivan denied granting immunity to Bulger or any other members of his Winter Hill gang in 2003 congressional testimony, according to court papers.
Bulger's case, which inspired Martin Scorsese's 2006 Academy Award-winning film "The Departed," stands as a black mark on the record of Boston law enforcement, with police of Irish descent collaborating with criminals who shared their ethnic background to undercut non-Irish gangs. (Reporting By Scott Malone; Editing by Paul Thomasch, Leslie Gevirtz and David Gregorio)