BOSTON (AP) — Lawyers for convicted gangster James “Whitey” Bulger asked a federal appeals court Monday to grant him a new trial, arguing that his defense was eviscerated when he was barred from testifying about his claim that he received immunity for his crimes.

Attorney Hank Brennan said a ruling by Judge Denise Casper prohibiting Bulger from telling the jury that a now-deceased federal prosecutor granted him immunity violated his right to a fair trial.

“The defendant has that right to testify. There is no shaking that right,” Brennan said.

But Assistant U.S. Attorney Randall Kromm told the three-judge panel of the 1st U.S. Circuit Court of Appeals that Casper only prohibited Bulger from testifying about his claim of immunity, finding that he had not produced any hard evidence that such an agreement existed. Kromm said that did not prevent Bulger from testifying at all.

“He chose not to testify,” Kromm said.

Bulger, the longtime leader of a violent gang that made millions from extorting drug dealers, bookmakers and others, spent 16 years as one of the nation’s most wanted fugitives before being captured in Santa Monica, California, in 2011. He claimed that former prosecutor Jeremiah O’Sullivan promised him immunity in exchange for Bulger protecting him against mobsters he prosecuted.

Brennan also argued that federal prosecutors failed to turn over certain details about an agreement they had with one of the key witnesses against Bulger, hit man John Martorano. Brennan said in addition to getting what he called an “unfathomable” deal from prosecutors for testifying against Bulger, Martorano was promised that he would not have to testify against his own family and friends.

The judges appeared skeptical about that claim, noting that Bulger’s lawyers never asked the trial court to make a ruling on whether there was such an agreement.

Judge William J. Kayatta Jr. noted that jurors were told about the plea agreement Martorano had with prosecutors — Martorano served 12 years in prison for killing 20 people — and asked what difference it would have made if they had been told that he also received other benefits.

Bulger, now 85, is serving life in prison. He was not present for the hearing.

The court is expected to take several months to issue a ruling.

After the hearing, former Assistant U.S. Attorney Brian Kelly, one of the prosecutors at his trial, said Bulger could have testified about anything other than his claim of immunity.

“He didn’t want to testify because he knew he’d be cross-examined for days … now he’s trying to pretend it was some kind of legal error,” Kelly said.

The jury at Bulger’s trial convicted him of participating in 11 murders in the 1970s and ’80s, but found that prosecutors did not prove he was also involved in seven other killings. The jury made no finding on whether Bulger participating in the murder of Debra Davis in 1981.

Her brother, Steve Davis, listened in court to the arguments and later said he’s “almost hoping” the appeals court grants Bulger a new trial so he can get justice for his sister.

“I think all the victims’ families didn’t get a fair trial,” he said.

But Davis also said it would be difficult to sit through another trial for Bulger.

“The only thing I want to hear from him is the closing of his casket,” he said.