Federal prosecutors today renewed their attack on James “Whitey” Bulger’s claim that he had immunity from prosecution for crimes including murder, calling the notorious gangster’s contention “fantastical,” “absurd,” and incoherent.

“The defendant has failed to provide any affidavit or evidence to support the absurd notion that he entered into a legally binding agreement with [late federal prosecutor Jeremiah] O’Sullivan that immunized the defendant in perpetuity for any crime the defendant chose to commit,” federal prosecutors wrote.

“Even if one were to indulge the defendant’s fantastical claim, such an agreement would have been void ... as against public policy.”

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The brief, filed in federal court in Boston, comes one day after Bulger’s lawyers said in court papers that he is not asserting a license to kill by claiming O’Sullivan granted him federal immunity from prosecution.

Rather, defense lawyers said, the agreement only barred federal prosecutors in Massachusetts from charging Bulger criminally, as long as he kept up his end of the deal with O’Sullivan. But the pact did not bar state prosecutors from charging the gangster, according to his lawyers.

They have not said what Bulger, 83, agreed to do in exchange for federal immunity. He denies ever being an informant, despite evidence from prior court proceedings that he passed information to the FBI for years.

In today’s filing, prosecutors scoffed at Bulger’s attempt to clarify the purported immunity agreement, which his attorneys said did not amount to “prospective immunity,” or the freedom to commit any future crimes.

“His mysterious contention that he has never claimed ‘prospective immunity ’ (while seemingly claiming that his agreement covered the commission of subsequent crimes) illustrates the incoherence of his own claim,” prosecutors said.

The prosecutors said it would be a “waste of time” for the court and government to address the claim.

Bulger is charged in a sweeping federal indictment with 19 murders and is slated to go on trial in June. He is asking US District Court Judge Denise J. Casper to allow him to present his immunity defense to the jury, a request that her predecessor on the bench denied last month.