Chicago Mob History 101 from the FBI

An FBI agent gave jurors a lesson in the history of the Chicago mob during the trial of a former deputy U.S. Marshal accused of blabbing secrets to alleged mobsters.

Special agent Michael Maseth was the first witness in the trial of John Ambrose, 42.

Ambrose is accused of leaking information about Nicholas Calabrese, the government's star witness at the Family Secrets trial that targeted top members of the Chicago mob. He's denied the allegations.

Maseth's testimony Tuesday underscored how Calabrese put his life at risk by cooperating with the government and how significant his information was.

Maseth called Calabrese the most important organized crime figure to ever testify in the district and "perhaps the entire United States."

Thanks to CBS2Chicago