BOSTON (AP) — Former mobster James "Whitey" Bulger's claim that a federal prosecutor gave him immunity to commit crimes should be decided by a jury, not a judge, said his lawyer, who denied Wednesday that Bulger was an informant.

A federal prosecutor said at Wednesday's hearing that any immunity agreement Bulger claims he had with the government would be "void as a matter of law" if it included murder.

Both sides presented arguments to U.S. District Judge Richard Stearns, who is scheduled to preside at Bulger's June murder trial. Defense attorney J.W. Carney Jr. wants the claim to be included at the trial, while prosecutors say a judge should decide the issue before then.

Bulger, the former leader of the Winter Hill Gang, is accused of participating in 19 murders during the 1970s and '80s. He was also an informant for the FBI during that time, according to FBI files and testimony from his former cohorts — but his lawyer denied that Wednesday.

"James Bulger was never an informant for the FBI or for anybody else," Carney told reporters after the hearing. He didn't answer questions about why Bulger would have had immunity if he hadn't helped federal law enforcement officials as an informant. Carney said he planned to answer that during trial.

A spokesman for U.S. Attorney Carmen Ortiz declined to comment on Carney's remark.

Bulger, 83, fled Boston in 1994 and was one of the FBI's most wanted fugitives until his capture in Santa Monica, Calif., in 2011.

Bulger had claimed federal prosecutor Jeremiah O'Sullivan gave him immunity for crimes while Bulger was providing the FBI with information on local leaders of his gang's main rivals, the Mafia. O'Sullivan, who died in 2009, denied making an immunity deal with Bulger when he testified before Congress in 2002.

Assistant U.S. Attorney Zachary Hafer told the judge Wednesday that even if Bulger is to be believed, as a matter of law, an assistant U.S. attorney doesn't have the authority to grant anyone immunity to kill Americans.

"Any contract between Mr. O'Sullivan and Mr. Bulger — to the extent it contemplated murder — would be void as a matter of law against public policy," Hafer said. Continued...