Boston - The judge who blocked former Boston mob boss James "Whitey" Bulger from claiming a US prosecutor gave him immunity for his crimes violated his rights by preventing him from testifying at his 2013 racketeering trial, Bulger's lawyer said on Monday.The lawyer, Hank Brennan, went before a three-judge panel of the First Circuit US Court of Appeals in Boston to challenge the conviction of Bulger, 85, on charges including committing or ordering 11 murders in the 1970s and 1980s.The trial detailed Bulger's corrupt relationship with federal agents and prosecutors in Boston, who turned a blind eye to the Irish-American gangster's crimes in return for information they could use against the Italian-American mafia.Brennan said District Judge Denise Casper violated Bulger's rights by not allowing him to testify that a US attorney for Massachusetts, Jeremiah O'Sullivan, had offered Bulger immunity for his crimes, which also included extortion and drug dealing. O'Sullivan is now dead."The government successfully precluded the appellant from testifying at trial," Brennan said. "We simply wanted the opportunity to ask the appellant the same questions that the government asked."Federal prosecutors, represented by Assistant US Attorney Randall Kromm, said Bulger had not been prevented from testifying, but from presenting what they called inadmissible evidence."There is this argument that he was precluded from testifying, but he chose not to testify," Kromm said. "It's very difficult to judge in a vacuum because we don't know what he thought he would say."Several members of Bulger's notorious "Winter Hill" gang testified against their old boss during his trial under deals with federal prosecutors that protected them from arrest.Throughout his trial, Bulger denied having been an informant, or a "rat" in mob parlance, insisting that he had paid law enforcement officials for information he could use against criminal rivals without providing any of his own.He said O'Sullivan had offered him immunity in return for protection of his own life. 'He didn't want to testify'Brian Kelly, a former assistant US attorney who led the prosecution team at Bulger's trial and is now in private practice, denied that the gangster had been prevented from testifying."If he had testified, he would have been cross-examined for days about the series of murders that the jury found he committed," Kelly told reporters after the hearing. "He didn't want to testify."Steven Davis, whose sister Debra was among the people who Bulger was accused but not convicted of killing, said he believed prosecutors had given Bulger some sort of immunity."He had a verbal immunity with some FBI agents and the government, that's my opinion," Davis said. "They were protecting all their activities. And when murder came in the picture, they were so deep in with these criminals that they had to fix things, change things, cover up."Davis added that he was torn about Bulger's appeal, with part of him wanting to see the conviction overturned so that he could be tried again on murder charges. The jury issued a "no finding" decision on Debra Davis' killing, unable to determine whether Bulger was guilty or innocent.Bulger, whose brother William was the president of the Massachusetts state Senate, fled the city in 1994 on a tip that his arrest was imminent. He spent 16 years on the lam, many listed atop the FBI's "Most Wanted" list, before his capture in California in 2011.