Deputy US Marshal John Ambrose, through his attorney, does not deny sharing secret information that could have put a top government informant in danger.
The question for jurors: was it a benevolent boast to a family friend or a sinister attempt to curry favor with a mob boss?
The tapes are grainy. The sound: spotty. The conversation: in code. But it was during recordings of Chicago mob boss James Marcello and his brother Michael at a prison in Milan, Michigan that the feds first learned they had a leak.
On the stand on Tuesday, FBI agent Michael Maseth said the two men were discussing something they shouldn't have known: another mobster, Nick Calabrese had flipped.
"He was the most important organized crime figure that has ever testified in this district and perhaps the entire United States," the lawman told jurors.
In court, prosecutors played tapes of the Marcello's chats which appear to include details of Calabrese's cooperation and of his secret visit to Chicago in which Calabrese drove federal agents around town pointing out mob burial grounds.
Prosecutors say the information all came from one of the men assigned to protect Calabrese: Deputy US Marshal named John Ambrose.
The government says the Tinley Park resident passed along details of the Calabrese conversion to a mob associate who was a father figure to him.
Jurors today saw a 2003 FBI threat assessment on Calabrese. It says, in part: "Every single member or associate of the Chicago Outfit presents a threat to Nick Calabrese because he is cooperating against them."
The threat assessment closes with this dire conclusion: "Calabrese will be killed by members or associates of [the Chicago mob] as a result of his cooperation."
Ambrose's attorney counters the Marcello's could have learned details about Calabrese's conversations with the feds from someone else in law enforcement or the legal community. They ask: if John Ambrose intended to share secrets with the mob why wouldn't he have simply told them the location of the safe house where he was guarding Calabrese?
One of the men seen on that prison video: Michael Marcello who is expected to testify on Wednesday afternoon under a grant of immunity. He's the half brother of convicted mob boss James Marcello. But the immunity deal doesn't mean he'll be a cooperative witness for the prosecution.
What he'll say remains a mystery. Will he detail how he came to know the FBI's secrets? Or will he toy with his longtime government pursuers?