(CNN) -- Reputed former mob boss James "Whitey" Bulger did not have the right to commit more murders after striking a purported immunity deal with the FBI in the 1970s, a federal judge ruled Monday, knocking down a key part of Bulger's defense at his trial.
Bulger, the alleged former head of Boston's notorious Winter Hill gang, faces 19 murder charges, as well as charges including extortion, money-laundering and narcotics distribution.
The defense had initially moved to dismiss the case, saying Bulger was granted immunity by federal agents working to infiltrate Irish and Italian mobs in Boston three decades ago.
Judge Richard Stearns denied Bulger had "prospective immunity" -- that is, immunity to commit crimes made after his purported deal with the FBI.
Stearns left undecided whether Bulger had "historical immunity" for crimes committed before a deal. He gave both sides time to review evidence on that point.
Bulger's attorneys had also sought to have a jury decide on his immunity claim, arguing he would receive a fairer hearing, but Stearns refused.
Bulger's attorneys said he is prepared to detail the immunity deal "for past and ongoing crimes" at trial, and they said they will continue to fight for a jury trial.
"The federal government has done everything in its power over the past 25 years to cover up the relationship between James Bulger and federal law enforcement authorities," the attorneys wrote in a statement after the ruling.
"The federal government, including attorneys who worked for the (U.S. Justice Department) during this period, desperately want to conceal this sordid history from the jury, the victims, and the public. Today's decision is another step toward that goal."
The U.S. attorney's office declined to comment.
Regarding Bulger's claim of immunity, Stearns wrote that a license to kill is "beyond the pale and one unknown even in the earliest formalities of the common law."
He wrote the defense has provided a "paucity of information" about the deal that Bulger says he struck with Jeremiah O'Sullivan, then the head of the Justice Department's New England Organized Crime Strike Force.
Prosecutors submitted an affidavit from a former U.S. Justice Department official who says that if O'Sullivan did grant Bulger immunity from his past crimes, then it was without proper approval and against agency practice.
The affidavit is part of the evidence the two sides will be reviewing on historical immunity.
Bulger made headlines when he was arrested in June 2011 in Santa Monica, California, after being on the run for 16 years.
Before his sudden departure from Boston, he cooperated as an informant with disgraced ex-FBI agent John Connolly Jr., who is serving a 50-year sentence for second-degree murder and racketeering.
According to an indictment against Connolly filed in 2000, Bulger became his confidential informant in fall 1975.