Hitman Frank Calabrese, jailed for life after a long career of murder and extortion in the Chicago mob, has died in prison.
His son, Frank Jnr, said he died on Christmas Day, which was "one of the few days out of the year he was a good person".
Calabrese Jnr added: "I've never been comfortable he was locked up the way he was - but he needed to be. I am comfortable with the fact he's not suffering in there any more and that no one else has to suffer on the street."
Calabrese, 75 - known as "Frankie Breeze" - was an inmate at the Butner Federal Correctional Complex in the US state of North Carolina.
Calabrese's lawyer, Joseph Lopez, said he had been in poor health. "He was a sick man," he said. "He was on about 17 different medications, but was always a strong-willed individual."
At his trial in 2007, Calabrese was found responsible for seven gangland slayings, racketeering, extortion and illegal gambling.
Witnesses said Calabrese preferred to strangle his victims, then cut their throats to make sure they were dead.
Calabrese's son, as well as his brother Nicholas, provided key testimony against him during the long-running courtroom drama, which exposed the sinister inner workings of organised crime in Chicago. Calabrese Jnr, who joined his father's 26th Street Crew as a teenager, testified his father once "pulled out a gun and stuck it in my face and said, 'I'd rather have you dead than disobey me'." Found guilty with his four-co-defendants, Calabrese was sentenced to life in 2009. During closing arguments, he allegedly mouthed "You are a [expletive] dead man" in the direction of Markus Funk, the federal prosecutor.
Funk said that those hurt by Calabrese's criminal actions would probably find it hard "to muster much regret" over his death behind bars.
"Calabrese reigned as one of the nation's most prolific and feared mob killers, only to die alone and far from home on Christmas Day," Funk said.
"Whether this qualifies as poetic justice, or the sad final chapter of a largely squandered existence, is for others to decide."
Calabrese Jnr said his feelings over his father's death were "not the normal emotions of a son".
He said: "There was a good side to my father, but my whole issue with him was he had multiple personalities. I'm thinking about the good after everything we've been through."
Calabrese's other survivors include his second wife, Diane Cimino, his four other sons, Kurt, Nick, James and Dominick, and five grandchildren.