A lawyer for James "Whitey" Bulger has asked an appeals court to remove the judge set to preside over the Boston mobster's murder case.

The defense told the federal appeals court Tuesday that reasonable people might have a reasonable doubt about U.S. District Judge Richard Stearns' impartiality.

The defense claims Stearns has a conflict of interest because Bulger was an FBI informant when Stearns was a federal prosecutor. They say Bulger got an immunity deal from the FBI before Stearns headed the U.S. Attorney's Office criminal division.

But a government lawyer argued Tuesday that Stearns, who has rejected the appeals to recuse himself, didn't participate in and didn't know of any investigations involving Bulger.

The former boss of the Winter Hill Gang faces a June trial involving 19 murder counts.