(CNN) -- Reputed Boston mob boss and longtime fugitive James "Whitey" Bulger is the defendant in a sensational murder and racketeering trial, but proceedings Friday in federal court had one prosecutor choking up with emotion while defending the work of another law enforcement officer.
And that moment came before a jury has even been seated.
Assistant U.S. Attorney Fred Wyshak choked up during arguments in the case as he defended one of his main investigators, Massachusetts Lt. Steve Johnson.
Wyshak was responding to a defense claim that one of the key witnesses in the case may have been involved in further criminal activity since his release from federal prison in 2007, and that a member of the Massachusetts State Police claimed the investigation into that defense witness' activities was compromised.
In court, Bulger attorney Henry Brennan said the state trooper who complained felt "undermined by one of his superiors, Steve Johnson." Brennan says the trooper was "not only undermined....so discontent he made complaints."
The defense witness, John Mortorano, has admitted he killed 20 people as part of Bulger's criminal organization that ruled south Boston for decades. He was released as part of a plea deal in exchange for his testimony.
Wyshak acknowledged in court that the Massachusetts state police investigated claims from "sources" that Mortorano was again involved in criminal activity after he was freed. Wyshak said the investigation included "numerous interviews" and that the "allegations were unfounded," calling the accusations "a lot of smoke."
Wyshak said the information was turned over to the defense in May.
What apparently wasn't turned over was an additional internal state police investigation into Johnson's handling of the case.
Wyshak, addressing U.S. District Court Judge Denise Casper, became emotional as he defended Johnson, calling him, "one of the best law enforcement officers I know."
Wyshak said the the internal investigation found complaints against Johnson were "unsubstantiated" and that the "real villain is the state trooper" who he says "went off the deep-end" and denied the allegations when confronted.
Wyshak said that trooper faces disciplinary action.
The judge did not rule on whether the defense is entitled to documents pertaining to the internal investigation.
The conduct of law enforcement is key in the case since prosecutors allege a corrupt FBI agent aided Bulger in his criminal actives while Bulger himself was a government informant.
The Friday back-and-forth came as the two sides attempt to seat a 18-member jury, including six alternates, in the high-profile case.
One hundred and 50 potential jurors whose questionnaires passed muster will be further scrutinized on Monday.
Bulger was arrested in California in 2011 after 16 years on the run. Now 83, he's charged in the killings of 19 people during his days as the leader of south Boston's Winter Hill gang.
Federal prosecutors say Bulger led the Irish mob from the late 1970s through the mid-1990s. But after Bulger fled impending racketeering charges, investigators found he had been an FBI informant whose handler tipped him off about the charges -- a tale that became the inspiration for the Oscar-winning 2006 drama "The Departed."